Construction unit conversions
The handful of conversions that actually get used, on one page — especially kN/kgf and MPa/kgf/cm², where most errors happen.
| Quantity | Conversion | Where it matters |
|---|---|---|
| Length | 1 m = 1000 mm = 100 cm = 3.2808 ft = 39.370 in | Drawings in mm, bills in m |
| Length | 1 in = 25.4 mm | Pipe and section sizes |
| Area | 1 m² = 10.7639 ft² = 10000 cm² | Imported material priced per ft² |
| Volume | 1 m³ = 1000 L | Concrete, mortar, water |
| Volume | 1 m³ = 35.3147 ft³ = 1.30795 yd³ | US drawings and equipment |
| Mass | 1 t = 1000 kg = 2204.6 lb | Steel and aggregate by the tonne |
| Force | 1 kN = 1000 N = 101.97 kgf ≈ 0.102 tf | Structural loads are in kN |
| Force | 1 kgf = 9.80665 N | Older drawings use kgf |
| Stress | 1 MPa = 1000 kPa = 1 N/mm² = 10.197 kgf/cm² | C30 means 30 MPa, about 300 kgf/cm² |
| Pressure | 1 atm = 101.325 kPa = 1.033 kgf/cm² | Pumps and compressed air |
| Power | 1 kW = 860 kcal/h = 1.3596 hp | HVAC and plant selection |
| Energy | 1 kcal = 4.1868 kJ, 1 kW·h = 3600 kJ | Energy conversion |
| Slope | 30° = 57.74% = 1:1.732 | Codes use percent |
| Flow | 1 m³/h = 0.2778 L/s = 16.667 L/min | Pumps and pipe sizing |
All values are computed from the stated formulas; ranges show the usual spread. Use measured data or the local code where it differs.
How to use this table
- Convert everything to base units (m, kg, s, N) before calculating, and do not skip steps.
- Structural work uses kN and MPa only — mixing in kgf is the single most common comment at design review.
- When a drawing shows 1:n, convert to percent before comparing with a code limit.
- Before pricing, normalise drawing units: European and American drawings use psi, lb and ft, so convert to MPa, kg and m before applying local schedules — never convert mid-formula.
- Carry one extra digit through intermediate results and round only at the end. Rounding at every step accumulates 2-3% error over five steps.
- Write units after the number, not inside the formula — 'calculate first, guess the units later' is the leading cause of conversion errors.
Common misuse
- Treating kN as kgf — a factor of 9.8, which loses an order of magnitude in the load.
- Treating MPa as kg/cm² — a factor of 10.2, which makes concrete appear ten times stronger.
- Reading a 2% fall as 2° — 2% is 1.15°, nearly a factor of two.
- Missing a factor of 10³ on area. 1 m² is 10⁶ mm², not 1000 mm². In the reinforcement ratio A_s/(b·h0) with everything in millimetres, one slip is a factor of 1000.
- Treating kN/m² as kg/m². 1 kN/m² ≈ 102 kg/m², so converting load from kN to kg multiplies by 102, not divide by 9.8 — mathematically equivalent, but 9.8 has a habit of becoming 98.
Calculate the actual number
Other reference tables
FAQ
How many kg is 1 kN?
About 102 kgf. So a floor rated at 200 kg per square metre is roughly 2 kN/m².
How many kgf/cm² is 1 MPa?
10.197, rounded to 10 on site. C30 concrete is 30 MPa, about 300 kgf/cm².
What angle is a 2% fall?
arctan(0.02) = 1.15°, i.e. 20 mm per metre.
How many newtons is 1 kg?
9.8 N at standard gravity (9.80665). Estimating at 10 N is 2% conservative and fine for budget work, but structural and lifting checks must use 9.8.
How many horsepower is 1 kW?
1 metric horsepower is 0.7355 kW and 1 imperial horsepower 0.7457 kW. Motors are rated in kW and diesels often in horsepower, 1.4% apart — size the margin on the smaller, metric figure.
How many kW is one refrigeration ton?
1 US refrigeration ton = 3.517 kW and 1 Japanese ton = 3.861 kW. Air-conditioning selection commonly uses US tons while datasheets mix kW and tons, so normalise before pricing.
Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.