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BTU Explained for Gas Tankless
Quick answer: BTU/h is a gas tankless heater's input rating. Required BTU/h ≈ GPM × temperature rise × 500 ÷ efficiency. Example: 5 GPM at a 70°F rise with 0.95 efficiency needs about 184,000 BTU/h, which is why whole-home gas units are often rated 150,000–199,000 BTU/h. Your installer confirms gas supply.
Context: what a BTU actually measures
A British Thermal Unit (BTU) is the energy needed to raise one pound of water by one degree Fahrenheit. Gas water heaters are rated in BTU per hour (BTU/h) of input: how much gas energy the burner can use at full fire. A tankless unit has no stored hot water to fall back on, so its BTU/h rating directly limits how many gallons per minute it can heat by a given amount.
That makes BTU/h one of the three numbers that matter most for gas tankless sizing, alongside GPM and temperature rise. It also drives the gas supply requirement — a big burner needs an adequately sized gas line and meter.
Terms
- Input BTU/h: gas energy consumed at maximum firing.
- Output BTU/h: heat delivered into water = input × efficiency.
- Efficiency (UEF or thermal efficiency): condensing units often ~0.90–0.97; non-condensing ~0.80–0.85.
- Modulation / turndown: the burner's ability to fire lower for small draws, for example 15,000–199,000 BTU/h.
- The 500 factor: 8.33 lb per gallon × 60 minutes ≈ 500, which turns GPM into pounds per hour.
The formula
Required input BTU/h ≈ GPM × rise (°F) × 500 ÷ efficiency.
Rearranged, the maximum GPM a unit can deliver ≈ BTU/h × efficiency ÷ (rise × 500).
Try it in the BTU calculator or start from flow in the GPM calculator.
Reference table (example, 0.95 efficiency)
| GPM | 50°F rise | 70°F rise | 90°F rise |
|---|---|---|---|
| 3 | 79,000 | 111,000 | 142,000 |
| 5 | 132,000 | 184,000 | 237,000 |
| 7 | 184,000 | 258,000 | 332,000 |
| 9 | 237,000 | 332,000 | 426,000 |
Numbers above ~199,000 BTU/h are beyond most single residential units, which is why some large homes use two units linked together.
Process: sizing with BTU/h
- Add your peak simultaneous flow (see GPM explained).
2. Find your winter rise with the temperature rise calculator.
3. Calculate required BTU/h with the formula above.
4. Compare with model specifications at your rise, not just the headline rating.
5. Ask your licensed installer to confirm the gas line and meter can supply that input along with your furnace, stove and dryer.
Worked example
Figures below are illustrative examples (2026-10-29), not quotes or guarantees.
Assumptions: two showers at 2.0 GPM plus a bathroom tap at 1.0 GPM = 5 GPM. Inlet 50°F, target 120°F, rise 70°F. Condensing unit, efficiency 0.95.
- 5 × 70 × 500 = 175,000 BTU/h output needed
- 175,000 ÷ 0.95 ≈ 184,000 BTU/h input
A 199,000 BTU/h model would cover this with a small margin. A 150,000 BTU/h model would deliver only about 150,000 × 0.95 ÷ (70 × 500) ≈ 4.1 GPM at that rise — enough for two showers but not three fixtures.
Mistakes to avoid
- Reading the headline BTU/h without applying efficiency.
- Using summer inlet temperatures.
- Forgetting that other gas appliances share the same supply.
- Assuming propane and natural gas units are interchangeable — they need the correct model or a certified conversion. See propane tankless.
FAQ
Is a higher BTU/h always better?
Only if you need the flow. Oversizing raises cost and gas-line requirements; good modulation keeps small draws efficient.
Why does my gas line matter?
A 199,000 BTU/h burner can need a larger line than a tank's ~40,000 BTU/h. Your installer checks size and length.
What about electric units?
They are rated in kW; 1 kW ≈ 3,412 BTU/h. See electric vs gas.
Does altitude matter?
Yes, high altitude can reduce effective output; check the manufacturer's derating guidance.
Safety and code reminders
Gas, venting, combustion air, condensate disposal and high-amperage electrical work all carry real safety risks, including carbon monoxide, fire, scalding and shock. They are also governed by local code and permits that differ between towns and provinces or states. Treat every step on this page as preparation for a professional visit, not as instructions to do the work yourself. Keep the manufacturer's manual for your exact model, set water temperature with scald risk in mind (120°F is a common household target), and stop and call a licensed professional whenever something is unclear.
Quick reference: typical fixture flow rates
These example flow rates help you translate everyday use into the GPM numbers used throughout this site. Check the rating stamped on your own fixtures, as older fixtures often run higher.
| Fixture | Example hot-water flow |
|---|---|
| Low-flow showerhead | 1.5–1.8 GPM |
| Standard showerhead | 2.0–2.5 GPM |
| Bathroom faucet | 0.5–1.2 GPM |
| Kitchen faucet | 1.0–1.8 GPM |
| Dishwasher | 0.8–1.5 GPM |
| Washing machine (warm) | 1.0–2.0 GPM |
| Bathtub fill | 2.0–4.0 GPM |
Add only the fixtures that genuinely run at the same time. Running the dishwasher or laundry outside shower hours is the simplest way to stay within a unit's capacity.
Questions to ask a licensed installer
Bring these questions to every quote so you can compare them fairly. Good installers welcome them, and the answers often reveal hidden costs before work starts.
- What peak GPM and temperature rise did you size this unit for, and what inlet water temperature did you assume for winter?
- Is my existing gas line, meter or electrical panel large enough, and if not, what exactly needs upgrading and what does that line item cost?
- What venting route will you use, how long is it, and does it meet the manufacturer's instructions and local code?
- Does the quote include a permit, isolation (service) valves for future flushing, a condensate drain if needed, and removal of the old heater?
- What are the labour warranty and the manufacturer's heat-exchanger and parts warranty terms, and do they require registration or yearly professional service?
- How long will the install take, and will there be a period without hot water?
How to read the numbers on this site
Every figure on TanklessMath is a planning estimate. Ranges reflect typical North American examples as of 2026-10-29 and will differ with your region, home, fuel prices and installer. Equipment ratings vary by model, so always check the specification sheet for the exact unit you are considering. When our example and a licensed installer disagree, trust the installer who has seen your home.
Next steps
Write down your peak fixtures, coldest incoming water temperature, fuel type and possible locations, then ask at least two licensed installers to confirm sizing, venting, gas or electrical capacity, permits and local code. Compare written quotes line by line so the unit, labour, materials and upgrades are separated. Questions about this site: hello@charlandandco.com.
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