Calculate heater resistance in ohms from voltage and wattage or voltage and current, or find a heating element’s power and current from its resistance.
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Heater Resistance Formula
The calculator uses Ohm’s law and resistive-load power equations. Resistance from voltage and power also supports balanced three-phase wye and delta heaters with three equal elements. AC inputs are RMS values and assume unity power factor.
Choose the named voltage/power, voltage/current, power/current, or voltage/resistance pair. The single-element and single-phase relationships are:
- R = heater resistance in ohms (Ω)
- V = supply voltage in volts (V)
- I = current draw in amps (A)
- P = heater power in watts (W)
Resistance from voltage and power: Enter the voltage and power for the same operating condition. For DC or single-phase heaters, R = V² / P and I = P / V. Rated values imply operating resistance, not a cold-resistance prediction. For three-phase heaters, select wye or delta under Supply configuration and enter line-to-line RMS voltage and total power.
Other modes: Select resistance from voltage/current or power/current, or power and current from voltage/resistance. These modes apply to a single element or single-phase resistive load.
Example Heater Values and Load Arithmetic
These resistive-load examples illustrate arithmetic. Equipment ratings, installation instructions and applicable electrical rules determine circuit suitability; the calculator does not size protection or conductors.
| Heater | Voltage | Power | Resistance | Current |
|---|---|---|---|---|
| Small space heater | 120 V | 750 W | 19.2 Ω | 6.25 A |
| Standard space heater | 120 V | 1500 W | 9.6 Ω | 12.5 A |
| Toaster element | 120 V | 1000 W | 14.4 Ω | 8.33 A |
| Water heater element | 240 V | 4500 W | 12.8 Ω | 18.75 A |
| Baseboard heater | 240 V | 2000 W | 28.8 Ω | 8.33 A |
| Electric clothes dryer | 240 V | 5000 W | 11.5 Ω | 20.83 A |
| Breaker | 80% of nominal amperage | Power at 80% and 120 V | Power at 80% and 240 V |
|---|---|---|---|
| 15 A | 12 A | 1440 W | 2880 W |
| 20 A | 16 A | 1920 W | 3840 W |
| 30 A | 24 A | 2880 W | 5760 W |
| 40 A | 32 A | 3840 W | 7680 W |
Example Problems
Example 1: 1500 W space heater on 120 V. Resistance is R = 120² / 1500 = 14400 / 1500 = 9.6 Ω. Current is I = 1500 / 120 = 12.5 A. This calculation alone does not establish an acceptable branch circuit. Follow the heater instructions and applicable installation rules.
Example 2: Unknown heater that pulls 8 A at 240 V. Choose resistance from voltage and current, then enter V = 240 and I = 8. Resistance is R = 240 / 8 = 30 Ω and power is P = 240 × 8 = 1920 W.
FAQ
Does this give cold or hot resistance? The calculator returns the operating resistance, which is what V² / P implies at rated conditions. Cold resistance depends on material and temperature; compare with the manufacturer’s resistance-versus-temperature data.
Why doesn’t my multimeter reading match the calculator? Resistance changes with alloy and temperature; there is no universal 5–15% correction or failure threshold. Compare with the manufacturer’s data and test procedure. Disconnect and verify de-energized equipment before resistance testing.
Can I use this for a 240 V heater wired in North America? Yes. Enter 240 V as the voltage. The math is the same; only the numbers change.
What if I only know the resistance and want the wattage at a different voltage? Choose power and current from voltage and resistance. Enter the resistance applicable at the intended temperature and the new voltage. The calculator returns the current and the power at that voltage using P = V² / R.
