Engineering Unit Conversions
Conversion factors for civil engineering and surveying, given to their full defined precision — with the definition each one rests on, and a clear mark against the units whose value genuinely varies by region.
How to read this page
Exact The factor is a definition, not a measurement. Since the international yard and pound agreement of 1959, the inch is defined as exactly 25.4 mm, and every imperial length, area and volume factor follows from it by arithmetic. These figures never need rounding at the source — round only at the end of your calculation.
Derived Exact in principle but conventionally quoted rounded, or dependent on a conventional constant such as standard gravity (9.806 65 m/s², itself exact by definition).
Regional The unit's size genuinely differs from place to place. Quoting one number for a bigha without naming the region is meaningless. These entries name the convention used.
Length
All imperial length units derive from the inch, defined as exactly 25.4 mm.
| Convert | To | Multiply by | Basis |
|---|---|---|---|
| inch | millimetre | 25.4 | Exact definition |
| foot | metre | 0.3048 | Exact — 12 in |
| yard | metre | 0.9144 | Exact — 3 ft |
| mile | metre | 1 609.344 | Exact — 5 280 ft |
| nautical mile | metre | 1 852 | Exact by international definition |
| US survey foot | metre | 0.304 800 609 601 219… | Exact — 1200⁄3937, deprecated |
| chain (Gunter's) | metre | 20.1168 | Exact — 66 ft |
| link | metre | 0.201168 | Exact — 1⁄100 chain |
The US survey foot and the international foot differ by 2 parts per million — trivial on a beam, serious on a coordinate. Over a 10 km baseline the difference is about 20 mm, and on a state plane coordinate of 2 000 000 ft it is roughly 1.2 m.
The United States formally deprecated the survey foot at the end of 2022 in favour of the international foot. Legacy data, older state plane coordinates and long-running projects may still be on it, so the unit has to be recorded, not assumed.
Area
| Convert | To | Multiply by | Basis |
|---|---|---|---|
| square foot | square metre | 0.092 903 04 | Exact — 0.3048² |
| square yard | square metre | 0.836 127 36 | Exact — 9 ft² |
| square inch | square millimetre | 645.16 | Exact — 25.4² |
| acre | square metre | 4 046.856 422 4 | Exact — 4 840 yd² |
| acre | hectare | 0.404 685 642 24 | Exact |
| hectare | square metre | 10 000 | Exact by definition |
| hectare | acre | 2.471 053 8… | Derived — reciprocal, non-terminating |
| square mile | square kilometre | 2.589 988 110 336 | Exact |
One acre is exactly 43 560 ft², a figure worth memorising because most of the South Asian land units below are defined as fractions of it.
South Asian land measure
These units are used daily in property and land survey across Pakistan, India, Bangladesh and Nepal. Some are exact fractions of an acre and travel safely; others vary by district and should never be converted without naming the local convention.
| Unit | In square feet | In square metres | Relationship | Status |
|---|---|---|---|---|
| Sarsai (sq karam) | 30.25 | 2.810 317 | 1⁄9 marla | Exact fraction of an acre |
| Marla | 272.25 | 25.292 853 | 9 sarsai; 1⁄160 acre | Exact — standard convention |
| Kanal | 5 445 | 505.857 053 | 20 marla; 1⁄8 acre | Exact |
| Acre | 43 560 | 4 046.856 422 | 8 kanal; 160 marla | Exact |
| Murabba (square) | 1 089 000 | 101 171.410 6 | 25 acre | Exact |
| Cent | 435.6 | 40.468 564 | 1⁄100 acre | Exact — South India |
| Guntha | 1 089 | 101.171 411 | 1⁄40 acre | Exact — Maharashtra, Karnataka |
| Ground | 2 400 | 222.967 296 | — | Exact — Tamil Nadu convention |
| Bigha | varies widely | varies widely | no single definition | Regional |
- The small marla. The standard marla is 272.25 ft² (one square karam × 9). Some housing schemes and localities use a "small marla" of 225 ft² (15 ft × 15 ft) — around 17% smaller. Always establish which is meant before pricing land.
- Bigha has no single value. It differs between and within states — from a few hundred to several thousand square metres. A conversion is only meaningful when the district's convention is stated alongside it.
Volume & capacity
| Convert | To | Multiply by | Basis |
|---|---|---|---|
| cubic foot | cubic metre | 0.028 316 846 592 | Exact — 0.3048³ |
| cubic metre | cubic foot | 35.314 666 7… | Derived — reciprocal |
| cubic yard | cubic metre | 0.764 554 857 984 | Exact — 27 ft³ |
| litre | cubic metre | 0.001 | Exact by definition |
| US gallon | litre | 3.785 411 784 | Exact — 231 in³ |
| imperial gallon | litre | 4.546 09 | Exact by definition |
- Treating US and imperial gallons as the same. The imperial gallon is about 20% larger — a material difference on any tank or water calculation.
- Using 35.31 for cubic metres to cubic feet in a long take-off. It is a non-terminating reciprocal; carry more digits and round at the end.
Mass & force
| Convert | To | Multiply by | Basis |
|---|---|---|---|
| pound (lb) | kilogram | 0.453 592 37 | Exact by the 1959 agreement |
| short ton (US) | kilogram | 907.184 74 | Exact — 2 000 lb |
| long ton (UK) | kilogram | 1 016.046 908 8 | Exact — 2 240 lb |
| tonne (metric) | kilogram | 1 000 | Exact by definition |
| kilogram-force (kgf) | newton | 9.806 65 | Exact — standard gravity |
| pound-force (lbf) | newton | 4.448 221 615 260 5 | Exact — lb × 9.806 65 |
| kilonewton (kN) | kilogram-force | 101.971 621… | Derived — 1000 ⁄ 9.806 65 |
A kilogram is mass; a kilogram-force is the weight of that mass under standard gravity. Loads in structural calculations are forces — kN, not kg — and the two are only interchangeable once 9.806 65 has been applied. Material densities are quoted in kg/m³, unit weights in kN/m³, and mixing the two is a frequent source of results out by a factor of about ten.
Pressure & stress
| Convert | To | Multiply by | Basis |
|---|---|---|---|
| N/mm² | MPa | 1 | Exact — identical units |
| MPa | pascal | 1 000 000 | Exact |
| psi | pascal | 6 894.757 293 168 4… | Derived — lbf ⁄ in², exact as a fraction but non-terminating |
| psi | MPa | 0.006 894 757 293 2… | Derived — non-terminating |
| ksi | MPa | 6.894 757 293 2… | Derived — non-terminating |
| kgf/cm² | MPa | 0.098 066 5 | Exact |
| bar | pascal | 100 000 | Exact by definition |
| standard atmosphere | pascal | 101 325 | Exact by definition |
The identity 1 MPa = 1 N/mm² is worth internalising: concrete grades quoted as 20, 25 or 30 N/mm² are the same numbers in MPa, so no conversion is needed between a mix specification and a stress calculation.
Temperature, densities & unit weights
| Quantity | Relation or value | Note |
|---|---|---|
| Celsius → Fahrenheit | °F = °C × 9⁄5 + 32 | Exact |
| Celsius → Kelvin | K = °C + 273.15 | Exact |
| Water | 1 000 kg/m³ · 9.81 kN/m³ | Derived — at about 4 °C |
| Structural steel | 7 850 kg/m³ → 76.98 kN/m³ | Derived — usually quoted as 77 kN/m³ |
| Reinforced concrete | 25 kN/m³ (code value) | Derived — equivalent to 2 549 kg/m³, not 2 500 |
| Plain concrete | 24 kN/m³ (code value) | Derived — equivalent to 2 447 kg/m³, not 2 400 |
| Cement (bulk) | 1 440 kg/m³ | Derived — the basis of the 0.0347 m³ bag |
| Brick masonry | 1 900–2 000 kg/m³ | Regional — varies with brick and mortar |
The kN/m³ figures codes give for concrete are rounded design conventions, not conversions of a stated density. Multiplying a density of 2 400 kg/m³ by 9.806 65 gives 23.5 kN/m³, yet codes specify 24; 2 500 kg/m³ gives 24.5, yet codes specify 25. The rounding is deliberate and conservative.
Use the unit weight the governing code specifies for load calculations, and a measured or quoted density only where mass itself matters — transport, batching, or ordering by weight.
How these figures were produced
Every factor marked Exact was computed as an exact rational number from its defining relationship — the inch as 25.4 mm, the pound as 0.453 592 37 kg, standard gravity as 9.806 65 m/s² — rather than copied from another table, so the digits shown are correct as far as they go.
The South Asian land units were checked for internal consistency: 9 sarsai to the marla, 20 marla to the kanal, 8 kanal to the acre, 160 marla to the acre and 25 acres to the murabba all reconcile exactly against 43 560 ft².
Free to quote and link. Suggested citation: QSCivilCalc (2026). Engineering Unit Conversions. https://qscivilcalc.com/engineering-unit-conversions/
