Plumbing unit conversions: mm to inches, bar to psi, °C to °F
To convert mm to inches, divide by 25.4. To convert bar to psi, multiply by 14.504. To convert °C to °F, multiply by 1.8 and add 32. The reference tables below cover every unit conversion that comes up on a plumbing job — pipe sizes, pressure, temperature, flow rate, volume, and area.
Prefer to type a number and get the result instantly? Use the plumbing unit converter → It handles mm↔inch, bar↔psi, °C↔°F, L/min↔GPM, L↔gal, and m²↔ft² in both directions.
mm to inches — pipe size conversions
The formula is simple: inches = mm ÷ 25.4. The factor 25.4 is the exact international definition (1 inch = 25.4 mm, agreed in 1959), so there is no rounding in the conversion itself — only in how many decimal places you choose to keep.
One thing worth knowing before reaching for the table: pipe sizes marked in millimetres are usually the outside diameterof the pipe, while nominal imperial sizes (½", ¾", 1") are notthe actual outside diameter — they're a historical convention. A nominal ½-inch copper pipe has an outside diameter of 15.88 mm (close to 16 mm), not 12.7 mm. If you're trying to match a fitting, always confirm whether a given dimension is OD, ID, or nominal before ordering.
| mm | inches |
|---|---|
| 6 | 0.236 |
| 8 | 0.315 |
| 10 | 0.394 |
| 12 | 0.472 |
| 15 | 0.591 |
| 16 | 0.630 |
| 20 | 0.787 |
| 22 | 0.866 |
| 25 | 0.984 |
| 28 | 1.102 |
| 32 | 1.260 |
| 40 | 1.575 |
| 50 | 1.969 |
| 100 | 3.937 |
| 150 | 5.906 |
| 200 | 7.874 |
Values computed as mm ÷ 25.4, rounded to 3 decimal places. Reverse: inches × 25.4 = mm.
Bar to psi — pressure conversions
psi = bar × 14.504. The factor comes from the SI definition: 1 bar = 100,000 pascals, and 1 psi = 6,894.757 pascals, so 100,000 ÷ 6,894.757 = 14.5038. Rounding to 14.504 is accurate to four significant figures for everyday work.
To go the other way (psi to bar), divide by 14.504. Example: 60 psi ÷ 14.504 = 4.14 bar. Typical residential supply pressure runs 2–5 bar (29–73 psi). A pressure-reducing valve is usually set around 3–4 bar (43–58 psi) for most homes.
| bar | psi |
|---|---|
| 0.5 | 7.25 |
| 1 | 14.50 |
| 2 | 29.01 |
| 3 | 43.51 |
| 4 | 58.02 |
| 5 | 72.52 |
| 6 | 87.02 |
| 7 | 101.53 |
| 8 | 116.03 |
| 10 | 145.04 |
Values computed as bar × 14.5038, rounded to 2 decimal places. Reverse: psi ÷ 14.504 = bar.
°C to °F — temperature conversions
°F = (°C × 1.8) + 32. To reverse it (°F to °C): subtract 32, then divide by 1.8. The table below covers the temperatures that matter on a plumbing job — from cold supply to the scalding threshold.
As a rough cross-check: 0°C is freezing (32°F), body temperature is 37°C (98.6°F), and 100°C is boiling (212°F). Legionella bacteria multiply between roughly 20–45°C (68–113°F) and are killed above 60°C (140°F) — which is why hot water cylinders are commonly set to 60°C and domestic thermostatic mixing valves (TMVs) blend the output down to around 50°C (122°F) for scalding protection at the tap.
| °C | °F |
|---|---|
| 0 | 32 |
| 20 | 68 |
| 37 | 98.6 |
| 60 | 140 |
| 70 | 158 |
| 100 | 212 |
Formula: °F = (°C × 1.8) + 32. Exact — no approximation in the definition.
L/min to GPM — flow rate conversions
GPM = L/min ÷ 3.785. One US gallon is exactly 3.785,411,784 litres by definition, so dividing litres per minute by 3.785 gives US gallons per minute. To reverse (GPM to L/min), multiply by 3.785.
UK spec sheets and Australian equipment sometimes list flow in litres per minute; US and Canadian pump curves, fixture specs, and code tables use GPM. The conversion is exact — the only source of error is reading the flow correctly in the first place.
| L/min | US GPM |
|---|---|
| 1 | 0.264 |
| 5 | 1.321 |
| 10 | 2.642 |
| 15 | 3.963 |
| 20 | 5.283 |
| 30 | 7.925 |
| 40 | 10.567 |
| 50 | 13.209 |
| 100 | 26.417 |
Values computed as L/min ÷ 3.785411784, rounded to 3 decimal places. US gallon only (not imperial/UK gallon).
Volume and area conversions
Two more conversions that come up often: litres to US gallons (water heater capacity, tank sizing) and square metres to square feet (floor area, fixture counts).
Litres to US gallons (L to gal)
US gal = L ÷ 3.785. A 200-litre hot water cylinder holds 52.8 US gallons. To convert gallons to litres, multiply by 3.785.
| litres (L) | US gallons |
|---|---|
| 1 | 0.264 |
| 5 | 1.321 |
| 10 | 2.642 |
| 20 | 5.283 |
| 50 | 13.209 |
| 100 | 26.417 |
| 200 | 52.834 |
| 500 | 132.086 |
| 1,000 | 264.172 |
Square metres to square feet (m² to ft²)
ft² = m² × 10.764. Used when a fixture unit table or drain sizing chart lists area in ft² but you have the floor plan in metres.
| m² | ft² |
|---|---|
| 1 | 10.764 |
| 5 | 53.820 |
| 10 | 107.639 |
| 20 | 215.278 |
| 50 | 538.196 |
| 100 | 1,076.391 |
1 ft = 0.3048 m (exact), so 1 ft² = 0.09290304 m² and 1 m² = 10.76391 ft².
How to use these conversions on the job
The most common situation where these tables come up is when a fitting, spec sheet, or code table uses different units from your plans. A supplier catalogue might list pipe OD in inches while your drawing shows DN sizes in mm; a pump datasheet might show flow in L/min while the designer's sizing table uses GPM; a UK or Australian pressure gauge reads bar while an American sizing chart references psi.
For quick on-site work, the three formulas worth memorising are: divide mm by 25 to get a close-enough inch figure (true factor is 25.4, so the error is about 1.6%); multiply bar by 15 for a quick psi estimate — 3% high but close enough for a sanity check (e.g. 3 bar × 15 = 45 psi vs. the true 43.5 psi); and for Celsius to Fahrenheit, double and add 30 as a rough mental check (0°C → 30°F, accurate to ±2°F for the 0–20°C range; error grows to ±6°F by 40°C).
For anything that needs to be precise — ordering parts, sizing a water heater, writing a spec — use the exact figures from the tables above or the unit converter tool, which applies the exact conversion factors without rounding.
Common questions
- Is 14 mm 1 inch?
- No. 1 inch = 25.4 mm exactly, so 14 mm = 0.551 inches (14 ÷ 25.4). The nearest common imperial pipe sizes are ½ inch (12.7 mm) and ⅝ inch (15.875 mm) — 14 mm falls between them.
- What is 1 inch in mm exactly?
- 1 inch = 25.4 mm exactly. This is the international definition agreed in 1959, not a rounded approximation. Every mm-to-inch conversion uses this exact factor — there is no rounding error in the definition itself.
- Does 25 mm equal 1 inch?
- Not exactly. 25 mm ÷ 25.4 = 0.984 inches — about 0.4 mm short of 1 inch. A 25 mm pipe and a 1-inch pipe are different sizes with different fittings; they cannot be joined without a reducing coupler.
- How do you convert mm to inches?
- Divide by 25.4. Example: 150 mm ÷ 25.4 = 5.906 inches. To reverse it (inches to mm), multiply by 25.4: 6 inches × 25.4 = 152.4 mm.
Reference & education only. Not professional, engineering, or code-compliance advice. Estimates are based on published model codes; local amendments and your Authority Having Jurisdiction (AHJ) govern. Always verify against the current adopted code and a licensed professional before doing work.
Last reviewed 2026-07.