kW vs M3/hr — Understanding Gas Flow Rate

When working with gas appliances, two different physical quantities come into play. Kilowatts (kW) measure power — the rate of energy delivery or consumption at any given moment. Cubic metres per hour (m³/hr) measure volumetric gas flow — the physical volume of gas passing through a pipe or meter per hour. Gas Safe engineers need to convert between these regularly: a boiler's kW output rating must be converted to m³/hr to size the supply pipe, verify meter capacity, and calculate total site gas demand for multi-appliance installations.

0.08912
m³/hr
1 kW input = m³/hr (CV 39.5)
11.221
kW
1 m³/hr = kW input (CV 39.5)
2.674
m³/hr
30 kW output boiler (92% eff.)
U6
meter type
≤6 m³/hr — standard domestic

The kW to M3/hr Formula

The conversion uses the same Ofgem-mandated calorific value formula as all UK gas calculations. Since both kW and m³/hr are rate quantities (per hour), the time dimension cancels cleanly — making the formula structurally identical to the kWh-to-m³ conversion but applied to instantaneous power and flow.

⚙ kW to M3/hr Formula
m³/hr = kW(input) × 3.6 ÷ (CV × VCF)
→ Simplified at CV 39.5: m³/hr = kW(input) × 0.08912
→ From output kW: m³/hr = (kW(out) ÷ η) × 0.08912
kW(input)Gas energy input rate
× 3.6Converts kWh to MJ
÷ CVkWh per m³ (39.5 avg)
÷ 1.02264Ofgem VCF (fixed)
η = efficiencye.g. 0.92 for A-rated
🔄 M3/hr to kW Reverse Formula
kW(input) = m³/hr × CV × VCF ÷ 3.6
→ Simplified at CV 39.5: kW = m³/hr × 11.221
→ kW output = kW(input) × η
× CV × VCFEnergy per m³ of gas
÷ 3.6MJ to kWh conversion
× ηGives output kW from input
⚠ Efficiency Adjustment Formula
kW(input) = kW(output) ÷ efficiency
→ 30 kW output ÷ 0.92 = 32.61 kW input → 32.61 × 0.08912 = 2.906 m³/hr
A-rated (ErP A)91–94% typical 2026
B-rated86–90%
D-rated78–85%
G-rated (old)65–78%

How to Convert kW to M3/hr — Step by Step

1

Determine Input kW (Not Output)

Gas pipe and meter sizing must use the INPUT kW — the gas energy rate entering the appliance — not the heat output delivered to the system. Check the appliance data plate for "Max Gas Input" or "Max Input." If only output kW is available, divide by efficiency.

Boiler data plate examples: "Max heat output: 30 kW Max gas input: 32.6 kW" ← use 32.6 "Rated output: 35 kW Efficiency: 93.2%" ← calculate: 35 ÷ 0.932 = 37.6 kW input "28 kW combi" ← plate output only: 28 ÷ 0.92 = 30.4 kW input (assume 92%)
Never use the output kW directly for pipe sizing — this under-estimates gas demand and can result in undersized pipework and pressure drop issues.
2

Apply the Flow Rate Formula

Multiply input kW by 3.6 to get MJ/hr, then divide by (CV × VCF). At UK national average CV 39.5 and VCF 1.02264, the simplified factor is 0.08912 m³/hr per kW. Use the calculator above for any CV or efficiency value.

30 kW boiler output, 92% efficiency: Input kW = 30 ÷ 0.92 = 32.61 kW Gas flow = 32.61 × 3.6 ÷ (39.5 × 1.02264) = 117.39 ÷ 40.394 = 2.906 m³/hr
At CV 39.5, quick mental arithmetic: multiply input kW by 0.09 for a fast estimate (2% over-estimate — safe for initial sizing).
3

Apply Diversity Factor for Multiple Appliances

For installations with multiple gas appliances, calculate m³/hr for each individually, sum them, then multiply by the appropriate diversity factor from BS 6400 (domestic) or IGE/UP/2 (commercial). Diversity factors reflect that not all appliances run simultaneously at full load.

Domestic example: 30 kW boiler input (32.6 kW) → 2.906 m³/hr 5 kW gas hob (input, 100%) → 0.446 m³/hr 4 kW gas fire (input, 100%) → 0.357 m³/hr Total undiversified: 3.709 m³/hr Diversity factor (0.80): × 0.80 Design flow rate: 2.967 m³/hr
BS 6400 diversity factors for domestic: single appliance = 1.0; boiler + one other = 0.85–0.95; boiler + two others = 0.75–0.85. Always confirm with current edition of the standard.
4

Check Meter & Pipe Capacity

Compare your design flow rate against the installed gas meter capacity and pipe carrying capacity. A standard UK domestic U6 meter handles up to 6 m³/hr. Pipe capacity depends on diameter, length, fittings and operating pressure — use BS 6891 pressure drop tables for detailed sizing.

Design flow = 2.967 m³/hr U6 meter capacity = 6.0 m³/hr ✓ Adequate 22mm copper, 10m run ≈ 3.5 m³/hr capacity ✓ 15mm copper, 10m run ≈ 1.2 m³/hr ✗ Insufficient
This calculator provides flow rate only — it does not replace a full BS 6891 pressure drop calculation. Always verify pipe sizing for the specific installation using measured pipe lengths and fitting counts.

Worked Examples — kW to M3/hr

24 kW Combi
24 kW output · 92% eff.
2.326 m³/hr
Input: 24 ÷ 0.92 = 26.09 kW
26.09 × 0.08912 = 2.326 m³/hr
Meter: U6 adequate ✓
35 kW System
35 kW output · 91% eff.
3.428 m³/hr
Input: 35 ÷ 0.91 = 38.46 kW
38.46 × 0.08912 = 3.428 m³/hr
Meter: U6 adequate ✓
Multi-appliance
30kW + 5kW hob + 4kW fire
2.967 m³/hr
Total: 3.709 m³/hr
Diversity × 0.80
Design: 2.967 m³/hr
Old G-Rated
28 kW output · 70% eff.
3.564 m³/hr
Input: 28 ÷ 0.70 = 40.0 kW
40.0 × 0.08912 = 3.564 m³/hr
vs new: 28÷0.92 = 2.712 m³/hr
Commercial 100 kW
100 kW input · LNG CV 42.0
8.364 m³/hr
100 × 3.6 ÷ (42.0 × 1.02264)
= 360 ÷ 42.95
= 8.384 m³/hr
Reverse: M3/hr → kW
3.5 m³/hr (CV 39.5)
39.27 kW
3.5 × 39.5 × 1.02264 ÷ 3.6
= 3.5 × 11.221
= 39.27 kW input

UK Gas Appliance Demand Reference 2026

Gas demand in m³/hr for common UK residential and commercial gas appliances, calculated at CV 39.5 kWh/m³. Boiler figures use 92% efficiency for output-to-input conversion. Hob, fire and cooker figures are direct input ratings (100% efficiency).

18 kW Combi
19.57 kW input
1.743 m³/hr
Studio / 1-bed flat
24 kW Combi
26.09 kW input
2.326 m³/hr
1–2 bed flat
30 kW Combi
32.61 kW input
2.906 m³/hr
3-bed house (UK avg)
35 kW System
38.46 kW input
3.428 m³/hr
4-bed house
Gas Hob (4-burner)
3.0 kW input
0.267 m³/hr
All burners full
Gas Range Cooker
5.0 kW input
0.446 m³/hr
All burners + oven
Gas Fire (radiant)
4.0 kW input
0.357 m³/hr
Typical living room
Commercial Boiler
100 kW input
8.912 m³/hr
Office / retail

UK Gas Meter Capacity Reference 2026

UK domestic and commercial gas meters are classified by their maximum flow capacity in m³/hr. Selecting the correct meter is a regulatory requirement — an undersized meter will restrict gas flow and cause appliance lockout. Meter selection is also a function of minimum flow (meters have a minimum operating rate, typically 2% of maximum).

U6 Meter
Standard Domestic
6 m³/hr
≈ 67 kW input (CV 39.5)
Single boiler up to ~60 kW output. Standard meter for UK homes with 1–2 gas appliances. Covers 30 kW boiler + hob + gas fire comfortably.
U16 Meter
Large Domestic / Light Commercial
16 m³/hr
≈ 180 kW input (CV 39.5)
Multiple large appliances, large detached houses with high gas demand, small commercial kitchens. Required when total diversified demand exceeds 6 m³/hr.
U25 / U40 Meter
Commercial
25–40 m³/hr
≈ 280–449 kW input
Commercial boiler plant, industrial bakeries, restaurants and catering operations with multiple large burners and process heat requirements.
Meter Type Max Flow (m³/hr) Max Input kW (CV 39.5) Max Output kW (92%) Typical Application
U66 m³/hr67.3 kW61.9 kWStandard domestic, single boiler
U6 (with cooker)6 m³/hr67.3 kW61.9 kW30 kW boiler + hob + fire — marginal
U1616 m³/hr179.5 kW165.1 kWLarge homes, light commercial
U2525 m³/hr280.5 kW258.1 kWSmall commercial premises
U4040 m³/hr448.9 kW413.0 kWMedium commercial / industrial
U6565 m³/hr729.4 kW671.0 kWLarge commercial / industrial
U100100 m³/hr1,122 kW1,032 kWIndustrial plant / district heating

kW input = m³/hr × CV × VCF ÷ 3.6 at CV 39.5,VCF 1.02264. Output kW assumes 92% boiler efficiency. Meter selection must comply with current Gas Industry Unsafe Situations Procedure (GIUSP) and be specified by a Gas Safe registered engineer.

kW to M3/hr Conversion Table 2026

Full reference table from 1 kW to 500 kW input at UK average CV 39.5 kWh/m³. All values are gas input kW to m³/hr. For output kW ratings, multiply your boiler kW by (1 ÷ efficiency) first — e.g. 30 kW output ÷ 0.92 = 32.6 kW input.

Input kW M3/hr (CV 39.5) M3/hr (CV 38.5) M3/hr (CV 40.5) M3/hr (CV 42.0 LNG) kWh/hr consumed
1 kW0.0891 m³/hr0.0914 m³/hr0.0870 m³/hr0.0838 m³/hr1.000 kWh/hr
5 kW0.446 m³/hr0.457 m³/hr0.435 m³/hr0.419 m³/hr5.000 kWh/hr
10 kW0.891 m³/hr0.914 m³/hr0.870 m³/hr0.838 m³/hr10.00 kWh/hr
15 kW1.337 m³/hr1.371 m³/hr1.305 m³/hr1.257 m³/hr15.00 kWh/hr
18 kW1.604 m³/hr1.645 m³/hr1.566 m³/hr1.508 m³/hr18.00 kWh/hr
19.57 kW (18kW out / 92%)1.744 m³/hr1.788 m³/hr1.703 m³/hr1.640 m³/hr19.57 kWh/hr
20 kW1.782 m³/hr1.827 m³/hr1.740 m³/hr1.675 m³/hr20.00 kWh/hr
26.09 kW (24kW out / 92%)2.326 m³/hr2.385 m³/hr2.270 m³/hr2.186 m³/hr26.09 kWh/hr
28 kW2.495 m³/hr2.559 m³/hr2.435 m³/hr2.344 m³/hr28.00 kWh/hr
30 kW2.674 m³/hr2.742 m³/hr2.609 m³/hr2.512 m³/hr30.00 kWh/hr
32.61 kW (30kW out / 92%)2.906 m³/hr2.980 m³/hr2.836 m³/hr2.731 m³/hr32.61 kWh/hr
35 kW3.119 m³/hr3.199 m³/hr3.044 m³/hr2.931 m³/hr35.00 kWh/hr
38.04 kW (35kW out / 92%)3.391 m³/hr3.477 m³/hr3.309 m³/hr3.186 m³/hr38.04 kWh/hr
40 kW3.565 m³/hr3.655 m³/hr3.479 m³/hr3.349 m³/hr40.00 kWh/hr
42 kW3.743 m³/hr3.838 m³/hr3.653 m³/hr3.516 m³/hr42.00 kWh/hr
50 kW4.456 m³/hr4.569 m³/hr4.348 m³/hr4.186 m³/hr50.00 kWh/hr
60 kW5.347 m³/hr5.483 m³/hr5.218 m³/hr5.023 m³/hr60.00 kWh/hr
70 kW6.238 m³/hr6.397 m³/hr6.087 m³/hr5.860 m³/hr70.00 kWh/hr
80 kW7.129 m³/hr7.311 m³/hr6.956 m³/hr6.697 m³/hr80.00 kWh/hr
100 kW8.912 m³/hr9.139 m³/hr8.696 m³/hr8.372 m³/hr100.0 kWh/hr
150 kW13.37 m³/hr13.71 m³/hr13.04 m³/hr12.56 m³/hr150.0 kWh/hr
200 kW17.82 m³/hr18.28 m³/hr17.39 m³/hr16.74 m³/hr200.0 kWh/hr
300 kW26.74 m³/hr27.42 m³/hr26.09 m³/hr25.12 m³/hr300.0 kWh/hr
500 kW44.56 m³/hr45.70 m³/hr43.48 m³/hr41.86 m³/hr500.0 kWh/hr

Formula: m³/hr = kW × 3.6 ÷ (CV × 1.02264). Highlighted rows show common boiler output ratings converted to input kW at 92% efficiency. For non-condensing boilers at 70–80% efficiency, actual m³/hr will be 15–30% higher than values shown for same output kW.

⚙ Need Boiler Output → kWh Usage?

Convert your gas meter m³ readings to kWh for billing calculations, or check your gas bill cost.

🔥 M3 to kWh Calculator 💷 Gas Bill Calculator 🌡 BTU to M3 📊 Usage Calculator

kW to M3/hr by UK Region & CV 2026

Because calorific value varies by region and gas source, the same kW input generates a different m³/hr demand across the UK. For a 30 kW combi boiler input, the difference between Scotland's lower CV gas and South East pipeline gas is approximately 5% — significant for precise commercial pipe sizing but typically within safety margins for domestic work. Use the actual CV from your gas contract or Xoserve for critical engineering calculations.

UK Region / Gas Source CV (kWh/m³) Band m³/hr per kW 30 kW → m³/hr 32.6 kW → m³/hr 100 kW → m³/hr
UK North East England37.5Low0.09389 m³/hr/kW2.817 m³/hr3.061 m³/hr9.389 m³/hr
Scotland38.0Below Avg0.09264 m³/hr/kW2.779 m³/hr3.020 m³/hr9.264 m³/hr
North West England38.5Below Avg0.09139 m³/hr/kW2.742 m³/hr2.979 m³/hr9.139 m³/hr
Yorkshire & Humber39.0Average0.09021 m³/hr/kW2.706 m³/hr2.941 m³/hr9.021 m³/hr
Wales39.0Average0.09021 m³/hr/kW2.706 m³/hr2.941 m³/hr9.021 m³/hr
🇬🇧 UK National Average39.5Avg0.08912 m³/hr/kW2.674 m³/hr2.905 m³/hr8.912 m³/hr
East Midlands39.8Average0.08845 m³/hr/kW2.654 m³/hr2.883 m³/hr8.845 m³/hr
South West England40.0Average0.08800 m³/hr/kW2.640 m³/hr2.869 m³/hr8.800 m³/hr
West Midlands40.2Above Avg0.08756 m³/hr/kW2.627 m³/hr2.855 m³/hr8.756 m³/hr
London & South East40.5Above Avg0.08696 m³/hr/kW2.609 m³/hr2.834 m³/hr8.696 m³/hr
East of England41.0High0.08590 m³/hr/kW2.577 m³/hr2.800 m³/hr8.590 m³/hr
Norway / North Sea Pipeline41.5High0.08485 m³/hr/kW2.546 m³/hr2.766 m³/hr8.485 m³/hr
Qatar LNG (Qatargas)42.0LNG0.08381 m³/hr/kW2.514 m³/hr2.732 m³/hr8.381 m³/hr
Australia LNG44.5LNG0.07916 m³/hr/kW2.375 m³/hr2.581 m³/hr7.916 m³/hr
USA Sabine Pass LNG46.0LNG0.07653 m³/hr/kW2.296 m³/hr2.495 m³/hr7.653 m³/hr

m³/hr per kW = 3.6 ÷ (CV × 1.02264). 30 kW column = direct input kW (100% efficiency). 32.6 kW column = 30 kW output ÷ 0.92 efficiency. Source: Xoserve, GIIGNL, National Grid 2026. CVs update quarterly — use contract CV for commercial engineering.

Frequently Asked Questions

How do I convert kW to m3/hr for gas?
Formula: m³/hr = kW(input) × 3.6 ÷ (CV × VCF). At UK average CV 39.5 and VCF 1.02264, this simplifies to m³/hr = kW × 0.08912. Example: 30 kW input × 0.08912 = 2.674 m³/hr. If you have an output kW rating, divide by efficiency first: 30 kW output ÷ 0.92 = 32.6 kW input → 32.6 × 0.08912 = 2.906 m³/hr. Use the calculator above for any appliance, CV or efficiency value.
How many m3/hr is a 24kW boiler?
A 24 kW output combi boiler at 92% efficiency has a gas input of 26.09 kW. Flow rate = 26.09 × 0.08912 = 2.326 m³/hr at UK average CV. If reading from the boiler data plate input rating directly, use that figure instead. A U6 domestic meter (6 m³/hr capacity) comfortably handles a 24 kW boiler plus additional domestic appliances. Standard 22mm copper pipework is adequate for this flow rate in typical domestic layouts.
What is the formula for kW to m3/hr gas?
The formula to convert kW to m³/hr of natural gas is: m³/hr = kW × 3.6 ÷ (CV × VCF). Where CV = calorific value in kWh/m³ (UK average 39.5), VCF = volume correction factor (Ofgem standard 1.02264), and the 3.6 factor converts kWh to MJ. Simplified at CV 39.5: m³/hr = kW × 0.08912. Reverse formula: kW = m³/hr × CV × VCF ÷ 3.6 = m³/hr × 11.221 at CV 39.5.
How many m3/hr does a 35kW boiler use?
A 35 kW output boiler at 92% efficiency: input = 35 ÷ 0.92 = 38.04 kW → flow = 38.04 × 0.08912 = 3.391 m³/hr. At 93% efficiency: 35 ÷ 0.93 = 37.63 kW → 3.354 m³/hr. At 94%: 3.319 m³/hr. A 35 kW boiler is typical for a 4-bedroom house. A U6 meter (6 m³/hr) remains adequate for a single boiler of this size even with a gas hob added. 22mm supply pipework is usually sufficient depending on pipe run length — verify with BS 6891 pressure drop calculation.
What gas meter do I need for a 30kW boiler?
A 30 kW output boiler at 92% efficiency requires approximately 2.906 m³/hr gas flow. A standard U6 domestic meter (6 m³/hr capacity) is adequate for a single 30 kW boiler — even with additional domestic appliances (gas hob ≈ 0.27 m³/hr, gas fire ≈ 0.36 m³/hr), the diversified total remains within 4 m³/hr. A U16 meter is only needed if total undiversified appliance demand exceeds 6 m³/hr, which would require approximately 67+ kW of simultaneous gas input — uncommon in standard UK homes.
How do I calculate total gas demand for multiple appliances?
Step 1: Convert each appliance to m³/hr using m³/hr = kW(input) × 0.08912. Step 2: Sum all values. Step 3: Apply diversity factor per BS 6400. Example: 30 kW boiler (32.6 kW input → 2.906 m³/hr) + gas hob (3 kW input → 0.267 m³/hr) + gas fire (4 kW input → 0.357 m³/hr) = 3.530 m³/hr total. Diversity factor 0.80 → design flow = 2.824 m³/hr. U6 meter adequate ✓. Enter multiple values manually in the calculator above by summing kW inputs before applying diversity.
What pipe size do I need for a 30kW gas boiler?
For a 30 kW output boiler requiring ≈ 2.9 m³/hr gas input, general guidance per BS 6891: 22mm copper is the standard minimum for most domestic boiler connections. 15mm copper handles ≈ 0.8–1.2 m³/hr (adequate for gas hob only, not a boiler). 28mm copper handles 4–8 m³/hr (only needed for very long runs or multiple large appliances). Always perform a full BS 6891 pressure drop calculation using actual pipe length, number of fittings and available pressure — this calculator provides flow rate only and does not replace engineering calculations.
How do I convert m3/hr to kW for gas?
Reverse formula: kW(input) = m³/hr × CV × VCF ÷ 3.6. At CV 39.5: kW = m³/hr × 11.221. Example: gas meter reading showing 3 m³/hr flow = 3 × 11.221 = 33.66 kW gas input. To find boiler output kW, multiply by efficiency: 33.66 × 0.92 = 30.97 kW output. Switch to M3/hr → kW mode in the calculator above for an instant result with your actual CV and efficiency values.
What is the difference between kW input and kW output for a gas boiler?
kW output is the useful heat delivered to the heating system or hot water cylinder. kW input is the rate of gas energy consumed — always higher than output. The difference is the boiler's efficiency: output = input × efficiency. For pipe sizing and meter capacity, always use input kW. Modern ErP A-rated condensing boilers in 2026: 91–94% efficient. Older non-condensing boilers: 65–80%. A poorly maintained G-rated boiler consumes 30–40% more gas than a modern A-rated equivalent to deliver the same output kW — its m³/hr demand will be proportionally higher.