Electrical Calculate a value
Why the voltage at the end of the cable is lower than at the supply
Calculating voltage drop over the run, and why the cable size that satisfies current may not satisfy voltage.
Engineering Problem-Solving Library
Start from the engineering problem, not the formula
Every topic here answers a specific question an engineer actually has to answer, states what the method can and cannot do, and links to the tools that compute the numbers. Where a calculator is the right answer it is linked; where judgement is the right answer, that is said plainly instead of hiding behind a formula.
- Calculate a value Get a number you can defend, with the method and the limits shown.
- Understand a concept Understand why the method works and when it stops being valid.
- Choose between alternatives Compare real alternatives against a stated set of criteria.
- Diagnose a problem Find the cause of something that is going wrong on site.
- Follow a procedure Follow an ordered engineering method from data to a decision.
Start with a worked example
These are the questions that come up most often on site and in review. Each one shows the method, the numbers and the check that catches a wrong answer.
Geotechnical Understand a concept
What the ground under your foundation can actually carry
How to go from a soil description to an allowable bearing pressure, and how to tell whether bearing or settlement will actually govern your footing.
Also: Civil & Structural
Civil & Structural Calculate a value
Sizing culverts and storm drains without silting them up
Choosing a barrel that passes the design flow, and the self-cleansing velocity that stops it silted up within a few seasons.
Also: Environmental
Mechanical & Fluids Diagnose a problem
Why a pump is cavitating and what NPSH margin it needs
Working out the net positive suction head available at the pump eye, and why a few centimetres of margin matter.
Also: Environmental
Renewable Energy Calculate a value
Battery bank size and how long it should last
Converting daily demand and autonomy into stored energy, and why depth of discharge changes the answer so much.
Also: Electrical
What are you trying to do?
The same problem often needs a different kind of answer. Pick the one that matches what you are actually being asked for.
Calculate a value
Get a number you can defend, with the method and the limits shown.
Geotechnical Calculate a value
How much earth pressure a retaining wall actually faces
Active versus at-rest pressure, and why a wall that cannot move needs a substantially larger design thrust.
Also: Civil & Structural
Civil & Structural Calculate a value
Sizing a steel beam when stiffness, not strength, is the limit
Checking bending capacity and serviceability deflection together, and reading which of the two actually governs the member.
Civil & Structural Calculate a value
Checking a retaining wall will not overturn, slide or sink
The three limit states every retaining wall must satisfy, and which one usually fails first on a narrow base.
Also: Geotechnical
Civil & Structural Calculate a value
Sizing culverts and storm drains without silting them up
Choosing a barrel that passes the design flow, and the self-cleansing velocity that stops it silted up within a few seasons.
Also: Environmental
Civil & Structural Calculate a value
Setting out a horizontal road curve from a deflection angle
Tangent, external distance, middle ordinate and curve length, and which of them you actually stake on site.
Construction & QS Calculate a value
Taking off concrete volumes without mis-ordering the pour
Where concrete volume actually goes — including the parts that are habitually forgotten — and how wastage and batching affect the order.
Also: Civil & Structural
Construction & QS Calculate a value
Designing formwork for the pressure of fresh concrete, not hardened concrete
Why formwork is designed for placement rate and temperature rather than the weight of the finished slab, and where the hydrostatic cap applies.
Also: Civil & Structural
Construction & QS Calculate a value
Why rebar needs a development length, and how long it is
Bond-based development length, the Class A and B lap distinction, and what actually controls the length you detail.
Also: Civil & Structural
Construction & QS Calculate a value
Staircase concrete volume and how it splits up
Waist slab, steps and landing as separate volumes, and why the pour is usually smaller than people expect.
Construction & QS Calculate a value
Blockwork quantities: blocks, mortar and the waste you actually incur
Block counts per square metre, opening deductions, mortar volume and a realistic waste allowance.
Construction & QS Calculate a value
Plaster and screed volume, cement bags and sand
Converting area and thickness into wet volume, dry volume and the bag count you will actually order.
Construction & QS Calculate a value
Excavation and trench volumes with the working space included
Calculating excavation for trenches and pits including working space, which is usually larger than the structure itself.
Construction & QS Calculate a value
Asphalt quantities: from plan area to tipper loads
Converting area and layer thickness into compacted volume, tonnage and the delivery schedule behind it.
Also: Civil & Structural
Electrical Calculate a value
Why the voltage at the end of the cable is lower than at the supply
Calculating voltage drop over the run, and why the cable size that satisfies current may not satisfy voltage.
Electrical Calculate a value
Sizing a cable for both current and voltage drop
Two independent constraints that must both be satisfied, and recognising which one actually chose the size.
Mechanical & Fluids Calculate a value
Where the pressure in a pipe line actually goes
Separating frictional loss from fitting loss, and why small bore runs lose far more than intuition suggests.
Mechanical & Fluids Calculate a value
Sizing a heat exchanger from the log mean temperature difference
Why the arithmetic temperature difference gives the wrong area, and how the log mean fixes it.
Also: Chemical & Process
Mechanical & Fluids Calculate a value
Long pipe runs, thermal expansion and the movement you must allow
How far a run grows, why plastics move far more than steel, and when expansion loops stop being optional.
Also: Construction & QS
Mechanical & Fluids Calculate a value
Measuring flow over a weir, and knowing when the reading is wrong
Converting head over a crest to discharge for common weir shapes, and the conditions that invalidate the reading.
Also: Environmental
HVAC & Services Calculate a value
Getting a first-pass cooling load before running a full block load
A screening calculation from envelope U-values and internal gains, and the margin it still needs.
Also: Civil & Structural
HVAC & Services Calculate a value
Choosing duct size from airflow and a velocity you can live with
Turning airflow and a velocity budget into a standard duct size, and the rectangular equivalent that fits the ceiling void.
HVAC & Services Calculate a value
Working out the static pressure a fan has to overcome
Velocity, friction factor and pressure drop per 100 ft, and what that means for the fan duty to specify.
Also: Mechanical & Fluids
Plumbing & Fire Calculate a value
Sizing drainage pipe by fixture units rather than by flow
Why drainage is sized by a loading unit, and reading the code table to get the building drain.
Plumbing & Fire Calculate a value
Estimating water supply demand and the service pipe that carries it
Water supply fixture units converted to a probable peak demand, and the service size that follows.
Plumbing & Fire Calculate a value
Septic tank sizing and why retention time matters
Converting residents into daily flow, applying the required retention time and the sediment allowance.
Also: Environmental
Water & Desalination Calculate a value
What salt rejection means for permeate quality
Relating feed concentration, rejection percentage and permeate salinity, and reading the recovery calculator correctly.
Also: Chemical & Process
Environmental Calculate a value
How much stormwater storage a site actually needs
Comparing peak inflow against the permitted discharge and turning the difference into a storage volume.
Also: Civil & Structural
Environmental Calculate a value
When the ground can absorb the water, and when it cannot
Sizing infiltration storage and soakaways, and recognising the soils that will not drain.
Also: Geotechnical
Environmental Calculate a value
How much greywater a building actually produces, and can you use it
Quantifying the reusable fraction of household wastewater and the storage it needs.
Also: Plumbing & Fire
Environmental Calculate a value
How much water a landscape actually needs
Applying crop coefficient to reference evapotranspiration, then correcting for how efficiently water is delivered.
Renewable Energy Calculate a value
How many solar panels a load actually needs
From appliance energy to module count, with the derating that stops an array from under-delivering.
Also: Electrical
Renewable Energy Calculate a value
Battery bank size and how long it should last
Converting daily demand and autonomy into stored energy, and why depth of discharge changes the answer so much.
Also: Electrical
Renewable Energy Calculate a value
Charge controller sizing and getting the array and bank to agree
Controller current from array size and battery voltage, and what happens when the array is oversized for the bank.
Also: Electrical
Chemical & Process Calculate a value
Tank and storage volume calculations that account for actual shape
Vertical, horizontal and rectangular storage, with working freeboard and the resulting usable volume.
Also: Mechanical & Fluids
Understand a concept
Understand why the method works and when it stops being valid.
Geotechnical Understand a concept
What the ground under your foundation can actually carry
How to go from a soil description to an allowable bearing pressure, and how to tell whether bearing or settlement will actually govern your footing.
Also: Civil & Structural
Geotechnical Understand a concept
How much settlement is acceptable, and which kind governs
Distinguishing immediate from consolidation settlement, and why the damage limit is usually about differential movement rather than total.
Also: Civil & Structural
Geotechnical Understand a concept
How much load can a pile take, and which part is doing the work
Splitting pile capacity into shaft friction and end bearing so you can tell whether a longer pile actually helps.
Geotechnical Understand a concept
Reading soil test results so the numbers mean something
Converting water content, unit weight and specific gravity into void ratio, saturation and the unit weights every other calculation expects.
Civil & Structural Understand a concept
Why a column fails long before the steel reaches its yield strength
Euler and Johnson behaviour, the slenderness ratio, and recognising the member that will buckle rather than crush.
Electrical Understand a concept
Why a conduit cannot be filled completely, and how full is too full
Conduit fill percentages by conductor count, the reason they exist, and choosing the next conduit when the first does not fit.
Also: Construction & QS
Electrical Understand a concept
Why a low power factor costs money, and how much correction is needed
Converting kW and power factor into the reactive power to remove, and choosing a defensible target.
HVAC & Services Understand a concept
Sensible load versus latent load, and why the split decides the equipment
Separating the heat you feel from the moisture you remove, and why a humid climate shifts the balance sharply.
Chemical & Process Understand a concept
Setting up a mass balance and knowing when it does not close
Conservation of mass across a process, and the arithmetic checks that catch an error before design does.
Also: Water & Desalination
Civil & Structural Understand a concept
Knowing when a calculation is not a design
The difference between a screening number and a compliant design, and where professional review genuinely has to enter.
Also: Geotechnical Electrical HVAC & Services Mechanical & Fluids Plumbing & Fire
Civil & Structural Understand a concept
The unit mistakes that ruin engineering calculations
The conversions and dimension slips that produce plausible-looking but wrong answers, and how to catch them.
Also: Geotechnical Electrical Mechanical & Fluids HVAC & Services Plumbing & Fire Water & Desalination Environmental Renewable Energy Chemical & Process
Choose between alternatives
Compare real alternatives against a stated set of criteria.
Electrical Choose between alternatives
Choosing a transformer: why standard sizes matter more than the load
Applying diversity and growth to connected load, then selecting the next standard rating and understanding the utilisation.
Mechanical & Fluids Choose between alternatives
Finding the duty point instead of guessing a pump size
Reading a pump curve as a system curve, and matching the two to find where the system will actually operate.
Water & Desalination Choose between alternatives
Choosing an RO recovery rate without scaling the membrane
How recovery drives concentrate quality, and why pushing it higher trades water for scaling risk.
Water & Desalination Choose between alternatives
Choosing pretreatment so the membranes survive
Matching filtration and dosing decisions to the fouling and scaling risk of the raw water.
Also: Chemical & Process
Diagnose a problem
Find the cause of something that is going wrong on site.
Construction & QS Diagnose a problem
Balancing cut and fill, and what happens when the volumes do not match
How excavation, compaction and bulking change the numbers, and why a balanced site is almost never balanced on paper.
Mechanical & Fluids Diagnose a problem
Why a pump is cavitating and what NPSH margin it needs
Working out the net positive suction head available at the pump eye, and why a few centimetres of margin matter.
Also: Environmental
Plumbing & Fire Diagnose a problem
Sizing a sump pump for the storm that will actually arrive
Converting drainage area and rainfall intensity into inflow, then applying the margin a real storm will need.
Also: Environmental
Renewable Energy Diagnose a problem
Sizing an inverter when motors start, not just run
Continuous rating versus surge capability, and the load types that decide which governs.
Also: Electrical
Follow a procedure
Follow an ordered engineering method from data to a decision.
Construction & QS Follow a procedure
Getting from a calculator result to a bill of quantities
The path from an engineered quantity to a priced line item, and where quantities get lost between disciplines.
Also: Civil & Structural
Construction & QS Follow a procedure
Measuring what is already built before you design against it
Surveying an existing structure so the design assumptions match reality, and the dimensions most often wrong.
Also: Civil & Structural
Construction & QS Follow a procedure
Temporary works checks before a concrete pour
The checks that matter before concrete is placed, when the formwork is carrying load that will not be there later.
Browse by engineering discipline
Each discipline carries its own colour, icon and label across the whole library, so a subject stays recognisable wherever you meet it.
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Civil & Structural Engineering Structural capacity, serviceability, load paths, geometry and the materials that carry them. 18 topics - Geotechnical Engineering Ground behaviour, bearing, settlement, slopes, piles and the soil parameters that control them. 9 topics
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Construction Engineering & Quantity Surveying Estimating, quantities, temporary works, site method and the commercial reality of building it. 14 topics -
Electrical Engineering Load, conductor sizing, voltage behaviour, protection and the distribution that carries power. 11 topics -
Mechanical & Fluid Engineering Pumps, piping, pressure drop, heat transfer and the fluid behaviour that decides equipment duty. 10 topics - HVAC & Building Services Cooling and heating loads, air distribution, ventilation and the services that condition a space. 6 topics
- Plumbing & Fire Protection Water supply and drainage sizing, fixture loading, pumping and fire protection water systems. 7 topics
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Water Treatment & Desalination Treatment processes, membrane systems, mass balance and the plant that produces usable water. 5 topics - Environmental Engineering Stormwater attenuation, reuse, infiltration, pollution control and water returning to the ground. 10 topics
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Renewable Energy & Solar Off-grid and grid-tied solar sizing, storage, inverter duty and harvestable energy. 5 topics -
Chemical & Process Engineering Process design, mass and energy balance, unit operations and safe operating envelopes. 6 topics
Calculators behind these topics
These are the EngiMetric tools the library calls on. Each one names the engine that computes its result, so a figure taken from a topic and a figure taken from the tool cannot disagree.
- RO Recovery Calculator RO recovery and permeate flow with full water and salt balance closure, concentrate concentration and salt loads.
- RO Salt Rejection Calculator Calculate membrane salt rejection percentage from feed and permeate TDS concentrations.
- RO Permeate TDS Calculator Project expected permeate total dissolved solids based on feed water salinity and rejection specs.
- RO Concentrate Flow Calculator Brine/reject flow with water and salt balance closure, concentrate TDS, concentration factor and salt loads.
- RO Mass Balance Calculator Simplified TDS-based dissolved-solids balance for RO feed, permeate, and concentrate flow and quality checks.
- RO Scaling Risk & Pretreatment Analysis Screen CaCO3 scaling risk in RO feed and concentrate using the Langelier Saturation Index (ASTM D3739) and the Stiff & Davis Stability Index (S&DSI) for high-TDS and seawater feeds.
- Plant Mass Balance Assemble the whole plant as an ordered chain of unit operations: raw intake to product and waste, stage-to-stage product flow, external make-up, derived concentrate TDS by salt conservation, and plant-wide water and salt balance checks.
- Water Analysis & Treatment Selection Enter a feedwater analysis once: derive hardness, cation–anion charge balance, TDS by summation, ionic strength and saturation indices (LSI/RSI), then compare water-quality drivers against the treatment surfaces already built on the platform — a qualitative start, not a diagnosis.
- RO Antiscalant & Chemical Dosing Calculator Calculate antiscalant (or chemical) dosing rate and 100% active-equivalent flow from dose concentration and product strength.
- Pump Power Calculator Calculate hydraulic power, shaft power, and motor electrical power from head and flow rate.
- Engineering Unit & Water Quality Converter Convert flow rates and TDS concentration units (mg/L ↔ ppm) using the Site canonical unit engines (m³/day internal).
- Pump Head / Total Dynamic Head Calculator Compose total dynamic head from static lift, friction losses, minor losses, and residual pressure.
- Water Tank Storage Calculator Size cylindrical and rectangular water storage tanks from dimensions or daily demand and residence time.
- Concrete Volume Calculator Build a multi-item concrete quantity schedule for slabs, footings, beams, walls and columns, with category subtotals and an Excel export.
- Slab Concrete Volume Calculator Calculate concrete volume for rectangular and L-shaped slabs with slab count and wastage margins.
- Footing & Foundation Volume Calculator Calculate concrete required for spread (pad) footings and strip footings, per-footing and total volumes with ready-mix allowance.
- Construction Material Quantity Takeoff Calculator Quantity takeoff for bulk construction materials with wastage: aggregate, sand, cement, and general volume-mass conversions.
- Trench Excavation & Bedding Calculator Calculate trench excavation volume, pipe displacement, bedding material quantity, and backfill volumes.
Elsewhere on EngiMetric
How this library is maintained
Counts, filters and related links on this page are derived from the published inventory at build time, so they cannot drift from the actual content. Every worked example is executed against its authoritative engine during the build, and a mismatch fails the build rather than reaching a reader.