Quick Answer: Therms to Kilowatt-hours
One therm is approximately 29.31 kilowatt-hours (1 thm ≈ 29.31 kWh).
Conversely, 1 kWh ≈ 0.03412 thm.
Formula: kWh = therms (US) × 29.307107
Worked examples
- 1 thm ≈ 29.31 kWh
- 2 thm ≈ 58.61 kWh
- 5 thm ≈ 146.54 kWh
Method and limitations
This page uses the US therm: exactly 100,000 International Table BTU, which is 100,000 × 1,055.05585262 J = 29.307107 kWh. US gas utilities bill in therms. The UK therm used on older British gas bills is defined as 105,506,000 J = 29.307222 kWh — a difference of four parts per million, immaterial for billing but worth naming rather than leaving ambiguous. Multiply therms by 29.307107 for the US figure. Calculations use full-precision constants; answers in this box are rounded for readability (≈ marks a rounded value) and the calculator shows up to 9 significant figures. The therm here is the US therm, defined as 100,000 International Table BTU.
Each factor is exact by definition, derived from exact definitions, or a documented conventional value — see the methodology and sources.
Conversion Formula
This page uses the US therm: exactly 100,000 International Table BTU, which is 100,000 × 1,055.05585262 J = 29.307107 kWh. US gas utilities bill in therms. The UK therm used on older British gas bills is defined as 105,506,000 J = 29.307222 kWh — a difference of four parts per million, immaterial for billing but worth naming rather than leaving ambiguous. Multiply therms by 29.307107 for the US figure.
Why converting therms to kWh matters when comparing heating
US gas bills are in therms; heat pumps and all electricity are in kWh. A therm on this page is the US therm (100,000 BTU = 29.307107 kWh). That is the fuel energy delivered to your meter, not the heat delivered to your rooms: useful heat = therms × 29.307107 × boiler efficiency.
A heat pump's electricity use for the same heat is that useful heat divided by its seasonal COP (SCOP). Comparing the two costs therefore needs three of your own inputs — gas price, electricity price and appliance efficiency — rather than any national average. Illustrative example: 500 therms at 90 % boiler efficiency is about 13,190 kWh of useful heat; at an SCOP of 3 that heat would need about 4,400 kWh of electricity.
Run your own numbers in the heat pump savings calculator, or read the Heat Pumps guide for how COP, tariffs and installation cost interact.
Constants and their sources are listed on the methodology page.
Complete Guide to Therms to Kilowatt-hours Conversion
What is a therm?
A therm is 100,000 BTU of energy. US gas utilities bill in therms; older British bills also used therms (with a slightly different UK definition) before UK bills moved to kWh.
What is a kilowatt-hour?
A kilowatt-hour (kWh) is one kilowatt sustained for one hour and the universal unit of electricity billing, battery capacity and solar production.
How the Conversion Works
Because one BTU is 1,055.06 J and one kWh is 3,600,000 J, 100,000 Btu(IT) works out to 29.307107 kWh. Gas meters measure volume; the utility multiplies by a calorific value to arrive at therms, then bills that figure.
Why This Conversion Matters
Comparing gas heating with electric heating or a heat pump needs both in the same unit first, then each adjusted: gas energy × boiler efficiency gives delivered heat, and delivered heat ÷ SCOP gives a heat pump's electricity use.
Scientific Context
The energy content (calorific value) of natural gas per cubic metre or cubic foot varies with its composition. Where gas is metered by volume, utilities apply a stated calorific value to convert volume into therms or kWh, which is why the same volume can produce slightly different energy readings month to month.
Step-by-Step Conversion Guide
Follow these steps to convert therms to kilowatt-hours manually:
Start with your value in therms
Example: You want to convert 5 thm
Apply the conversion formula
Use the formula: kWh = therms (US) × 29.307107
Calculate the result
5 thm = 146.5355 kWh
Verify your answer
Use our calculator above to double-check your manual calculation.
Quick Mental Math Tip
For a quick estimate, multiply therms by 30 and subtract about 2.3%: 100 therms ≈ 3,000 − 69 = 2,931 kWh (exact: 2,930.71).
Common Conversions
| Therms (thm) | Kilowatt-hours (kWh) | Kilowatt-hours → Therms |
|---|---|---|
| 1 thm | 29.3071 | 1 kWh = 0.0341214 thm |
| 2 thm | 58.6142 | 2 kWh = 0.0682428 thm |
| 5 thm | 146.536 | 5 kWh = 0.170607 thm |
| 10 thm | 293.071 | 10 kWh = 0.341214 thm |
| 20 thm | 586.142 | 20 kWh = 0.682428 thm |
| 30 thm | 879.213 | 30 kWh = 1.02364 thm |
| 50 thm | 1,465.36 | 50 kWh = 1.70607 thm |
| 75 thm | 2,198.03 | 75 kWh = 2.55911 thm |
| 100 thm | 2,930.71 | 100 kWh = 3.41214 thm |
| 250 thm | 7,326.78 | 250 kWh = 8.53035 thm |
| 500 thm | 14,653.6 | 500 kWh = 17.0607 thm |
When You'll Need This Conversion
Gas versus heat pump decisions
Convert your gas bill into kWh to compare it directly against electric heating costs.
Mixed-fuel energy totals
Express gas and electricity use in one unit (kWh) to see a building's total energy use.
Annual heat demand estimate
A year of therms × 29.307107 × boiler efficiency estimates the annual heat (kWh) a replacement must supply. Equipment size (kW) needs a heat-loss calculation, not annual energy.
Energy literacy
Understand what a utility statement actually charges for.
Pro Tips for Accurate Conversions
Always adjust for efficiency
Multiply gas kWh by your boiler's seasonal efficiency, from its datasheet or energy label, to get delivered heat before comparing with a heat pump.
Use a whole heating season
Single-month comparisons are distorted by weather. Twelve months of therms gives a usable baseline.
Check the calorific value on your bill
It shifts by a few percent between suppliers and seasons, which changes your therm total.
Common Mistakes to Avoid
Comparing gas and electricity unit prices directly
A therm contains 29.3 times the energy of a kWh, so gas always appears cheaper per unit.
Convert to a common unit, then divide by appliance efficiency or COP.
Assuming 100% boiler efficiency
No boiler turns all of the gas energy into useful heat; part is lost in the flue gases.
Use the appliance efficiency rating printed on the unit.
Ignoring standing charges
Fixed daily gas charges continue even after switching most heating to electricity.
Include standing charges in any switching calculation.
History & Background
Gas is billed by energy content rather than volume because the energy in a given volume of gas varies with its composition. The therm — 100,000 BTU — is the energy billing unit used in the US and, historically, in Britain.
Key Milestones
The International Table BTU (1,055.05585262 J) is adopted; the US therm is 100,000 of these.
Global Usage Today
US gas bills commonly use therms. Many European bills, including current UK bills, state gas in kWh; some Australian bills use megajoules.
Frequently Asked Questions
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Conversion factors on this page are either exact by definition, derived from exact definitions, or conventional values in documented use. Each is listed with its source in our conversion methodology and sources, which cites the BIPM SI Brochure, NIST Special Publication 811 and the relevant IEC or national standards. Results are rounded for display only; calculations use full precision.