Ohm's law calculator
Pick the two values you know (voltage and current, voltage and resistance, or current and resistance) and get the other two plus power.
Power
2300 W
- Voltage230 V
- Current10 A
- Resistance23 Ω
- Power2300 W
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How it's calculated (visible formula)
V = I × R; P = V × I = I² × R = V² ÷ R
- Net geometric measurement, no waste.
- Adjustable waste % for cuts and breakage.
- Purchase rounded up to sellable units (boxes, bags, buckets, boards).
How to calculate it, step by step
- Identify which two quantities you know (V, I or R).
- Choose that combination in the selector.
- Enter values in base units (V, A, Ω).
- Get the third with V = I × R.
- Get power with P = V × I.
Worked example: 12 Ω heater on 120 V
- I = 120 ÷ 12 = 10 A
- P = 120 × 10 = 1,200 W
→ 10 A and 1.2 kW.
Common mistakes
- Applying Ohm's law to motors without power factor.
- Using peak instead of RMS voltage.
- Mixing up mA and A.
Pro tips
- Measure resistance with the circuit de-energized.
- For AC motors, use impedance and power factor.
- Convert mA to A and kΩ to Ω before calculating.
- Check a resistor's wattage rating exceeds what it dissipates.
From calculation to quote
Add labour, overhead and your margin, and generate the professional PDF.
FAQs
- What is Ohm's law?
- Current equals voltage divided by resistance: I = V ÷ R.
- How do I calculate power?
- P = V × I, or I² × R, or V² ÷ R.
- Does it work for AC?
- For resistive loads yes; motors and LEDs need impedance and power factor.
- What resistance is a 1,500 W heater at 120 V?
- R = V² ÷ P = 120² ÷ 1500 ≈ 9.6 Ω.
- How many amps does a 10 W LED bulb draw?
- About 0.083 A at 120 V.
- What is an ohm?
- The resistance that passes 1 A with 1 V applied.
Sources and standards
Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.