Common Units in Civil Engineering — Conversion Tables & Worked Examples

Drawings arrive in feet and inches, design codes work in millimetres and megapascals, the batching plant thinks in cubic metres, and the landowner talks in marlas. A civil engineer who cannot move between these systems quickly will make mistakes — usually expensive ones.

This is a complete, verified reference: conversion tables for every quantity you will meet on site, plus worked examples showing how the conversions are actually used.

Length

From	To	Multiply by
1 inch (in)	millimetres	25.4
1 foot (ft)	metres	0.3048
1 yard (yd)	metres	0.9144
1 mile	kilometres	1.60934
1 metre (m)	feet	3.28084
1 kilometre (km)	miles	0.62137

Key site facts: 1 m = 1000 mm = 100 cm; 1 ft = 12 in; 1 yd = 3 ft.

Area (Including Pakistani Land Units)

From	To	Multiply by
1 ft²	m²	0.092903
1 yd²	m²	0.836127
1 m²	ft²	10.7639
1 acre	m²	4046.86
1 acre	ft²	43,560
1 hectare	m²	10,000
1 hectare	acres	2.47105

Pakistani land units (Punjab standard):

Unit	Equals
1 marla	272.25 ft² = 25.29 m²
1 kanal	20 marla = 5,445 ft² = 505.86 m²
1 acre	160 marla = 8 kanal = 43,560 ft²
1 murabba	25 acres

Worked example: A plot is 2 kanal. In square metres: 2 × 505.86 = 1,011.7 m². In marlas: 2 × 20 = 40 marla.

Volume

From	To	Multiply by
1 ft³	m³	0.0283168
1 m³	ft³	35.3147
1 yd³	m³	0.764555
1 litre (L)	m³	0.001
1 litre	ft³	0.0353147
1 US gallon	litres	3.78541
1 imperial gallon	litres	4.54609
1 m³	litres	1,000

Key site facts: 1 m³ = 1000 L; a standard concrete mixer truck carries 6–8 m³. Discharge is also volume per time: 1 cusec (ft³/s) = 0.02832 m³/s.

Worked example: A footing needs 4.5 m³ of concrete. In cubic feet: 4.5 × 35.3147 = 158.9 ft³. In litres: 4.5 × 1000 = 4,500 L.

Mass

From	To	Multiply by
1 pound (lb)	kg	0.453592
1 kg	lb	2.20462
1 metric tonne	kg	1,000
1 metric tonne	lb	2,204.62
1 US (short) ton	kg	907.185
1 maund (trade)	kg	40 (Pakistan’s standard trade maund; the historical British imperial maund was 37.32 kg)

Worked example: 1 m³ of normal concrete weighs about 2,400 kg = 2.4 tonnes = 2.4 × 2204.62 = 5,291 lb. This single number drives truck counts, crane picks, and dead-load estimates.

Rebar Weight (Used Daily on Site)

The weight of steel bars per metre follows one simple formula:

Weight (kg/m) = D² / 162, where D = bar diameter in mm

Bar diameter	Weight per metre
10 mm	0.617 kg/m
12 mm	0.888 kg/m
16 mm	1.579 kg/m
20 mm	2.466 kg/m
25 mm	3.853 kg/m
32 mm	6.313 kg/m

Worked example: A bar bending schedule calls for 85 m of 20 mm bars. Weight = 85 × 2.466 = 209.6 kg. The 162 in the denominator comes from the density of steel (7,850 kg/m³): it is exact enough for every site calculation. For lap length calculations using these bars, see the lap length guide.

Force

From	To	Multiply by
1 newton (N)	lbf	0.224809
1 kilonewton (kN)	lbf	224.809
1 kgf	N	9.80665
1 tonne-force	kN	9.80665
1 MN	kN	1,000

Key fact: 1 kg of mass weighs 9.81 N on Earth. Site engineers often say “10 tonnes” when they mean a 98.1 kN load — fine for talk, but use proper force units in calculations.

Pressure and Stress (The Most-Used Table in Design)

From	To	Multiply by
1 N/mm²	MPa	1 (exactly)
1 MPa	psi	145.038
1 psi	kPa	6.89476
1 kg/cm²	MPa	0.0980665
1 kg/cm²	psi	14.2233
1 kPa	kN/m²	1 (exactly)
1 bar	MPa	0.1
1 bar	psi	14.5038

Worked example 1: A cylinder test gives 32 N/mm². In MPa: 32 MPa. In psi: 32 × 145.038 = 4,641 psi.

Worked example 2: A US specification calls for 3,000 psi concrete. In MPa: 3000 × 0.00689476 = 20.7 MPa — roughly equivalent to M20 grade.

Worked example 3: A soil bearing capacity of 150 kN/m² = 150 kPa = 150 × 0.145038 = 21.8 psi.

Density and Unit Weight

From	To	Multiply by
1 lb/ft³	kg/m³	16.0185
1 g/cm³	kg/m³	1,000
1 kg/m³ (mass density)	kN/m³ (unit weight)	÷ 1,000 × 9.81

So concrete at 2,400 kg/m³ has a unit weight of 2,400 × 9.81 / 1,000 = 23.5 kN/m³ (commonly rounded to 24 kN/m³ in design). For the full list of material unit weights, see the unit weight guide.

Speed and Temperature

From	To	Multiply by
1 km/h	m/s	0.277778
1 mph	km/h	1.60934
°C to °F	—	°F = °C × 9/5 + 32
°F to °C	—	°C = (°F − 32) × 5/9

Worked example: Concrete temperature limit 35°C = 35 × 9/5 + 32 = 95°F.

Money and Counting (Pakistani Site Practice)

Unit	Equals
1 lakh	100,000
1 crore	10,000,000 (100 lakh)
1 arab	1,000,000,000 (100 crore)

Practical Tips

1. Keep mm and m straight. The single most common site arithmetic error is mixing millimetres and metres in one formula — a beam 300 mm wide is 0.3 m, not 300 m.
2. Write the units in every calculation. If the units cancel correctly, the number is usually right.
3. psi ↔ MPa shortcut: multiply psi by 0.007 for a quick mental MPa (≈ 3,000 psi ≈ 21 MPa).
4. The D²/162 rule for rebar weight and the 2,400 kg/m³ concrete figure should be memorised — you will use them weekly.
5. Land units vary by province — the marla above is the Punjab standard (272.25 ft²). Confirm the local standard before any transaction.

Frequently Asked Questions

How many marlas are in one acre?
160 marlas (Punjab standard). One acre = 8 kanal = 43,560 ft² = 4,046.86 m².

What is 1 N/mm² in psi?
1 N/mm² = 1 MPa = 145.04 psi.

How much does 1 cubic metre of concrete weigh?
About 2,400 kg, or 2.4 metric tonnes (23.5 kN/m³).

What is the formula for rebar weight per metre?
Weight (kg/m) = D²/162, where D is the bar diameter in millimetres. A 20 mm bar weighs 2.466 kg/m.

Final Thoughts

Bookmark this page — unit conversion is one of those skills that quietly underpins every estimate, every test result, and every drawing you read. When a conversion feeds into a structural calculation, double-check it; when it feeds into a purchase order, triple-check it.

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Written by Mohsin Raza Adil — BSc Civil Engineering Technologist (NTC-registered), Pakistan.