What the result means
Ohm's law ties voltage, current and resistance together, and the power law adds watts. With any two known values, the other two are fixed. That makes it quick to check a heating element, size a fuse or sanity-check a meter reading.
The results assume a resistive DC or AC load. Motors, compressors, transformers and electronics have power factor and inrush that make real current differ.
How it's calculated
- V = I × R
- P = V × I = I² × R = V² ÷ R
- I = V ÷ R = P ÷ V
- R = V ÷ I = V² ÷ P
Worked example
A heating element draws 10 A at 120 V.
- R = 120 ÷ 10 = 12 Ω
- P = 120 × 10 = 1,200 W
The element should measure about 12 Ω cold (often a little lower, since resistance rises with temperature).
Reference table
The values below are computed from the same calculator functions used above. They let you compare common sizes and operating points without replacing the interactive calculation or its assumptions.
| Solve for | Formula |
|---|---|
| Volts | V = I × R · V = P ÷ I |
| Amps | I = V ÷ R · I = P ÷ V |
| Ohms | R = V ÷ I · R = V² ÷ P |
| Watts | P = V × I · P = I² × R |
Reference values are examples based on the stated assumptions. Verify manufacturer data, conductor ampacity and local requirements before using a result.
Sources and assumptions
The listed formulas use the assumptions shown in the worked example. Calculator formulas are published with their units so a technician can check a result instead of treating it as a black box.
Last reviewed . Spot an error? Email support@tiermenu.com.
FAQ
Why must I enter exactly two values?
Two values define the other two. A third value could conflict, so the calculator asks you to leave two blank.
Does this work on AC?
For resistive loads like heaters and incandescent lamps, yes. For motors and electronics, include power factor using the amperage calculator.
Why does my element read lower resistance than calculated?
Most heating elements have lower resistance cold than at operating temperature.
Can I use it to size a breaker?
It gives current, but breaker sizing also depends on continuous-load rules and conductor ampacity in your code.
Using a reference result in the field
A calculator is most useful when the inputs match the job being checked. Where a field asks for a length, confirm whether it is one-way or total; where it asks for current, confirm whether the load is continuous or intermittent. The unit label beside the result is part of the result; do not carry a number into another calculation without checking its units.
Reference rows are provided for comparison, not as universal design limits. They use the values and conditions stated with each table. A different conductor material, temperature, power factor or load type can change a field result. Where the page calls out a code check, equipment rating or manufacturer table, that check remains part of the job.
Use the interactive inputs for the exact conditions at hand, then review the displayed formula and assumptions. If a value looks unexpected, change one input at a time and compare it with a known point in the reference table. Keep the measured readings and job-specific notes with the work record so another technician can understand how the result was obtained.
TierMenu provides these free calculators without an email gate. The results stay in this browser unless you choose to copy a shareable URL or print the page. A shared URL includes the calculator inputs in its query string; do not put personal or sensitive information in calculator fields.