Calculate resistor heat energy, current, resistance, or time with I²Rt, plus power, wattage margin, temperature-rise estimates, and unit conversions.

Assumes constant resistance and DC or RMS current over the interval. Optional temperature is a steady-state estimate; verify part, mounting and pulse limits.

Rating and temperature (optional)

Blank omits rating utilization. Use the allowable rating under your actual conditions.

Blank omits temperature. Use a value for the actual package, mounting and cooling.

Used only with thermal resistance; default is 25°C.

Understanding Resistor Heat

When current passes through a resistor, electrical energy is converted into heat. This calculator estimates the total heat energy dissipated over a selected time interval and reports it in joules (J), watt-hours (Wh) and kilowatt-hours (kWh). It also solves the inverse current, resistance and time equations. The relationship assumes constant resistance and DC current, or RMS current evaluated over the same interval. That makes it useful for estimating energy loss, checking long-duration resistor loading, and understanding how quickly a resistor turns electrical power into heat.

Core Equations

Quantity Formula Meaning
Power dissipation
P = I² × R
 Rate of heat generation in watts
Total resistor heat
RH = P × t = I² × R × t
 Total energy dissipated over time

For manual checks, use current in amps, resistance in ohms, and time in hours if you want the result in Wh. If time is entered in seconds or minutes, convert it to hours before checking the arithmetic by hand.

What Each Input Does

Input Typical units Effect on result
Current A, mA Squared effect: doubling current increases heat by 4×.
Resistance Ω, kΩ, MΩ Linear effect at fixed current and time: doubling resistance doubles heat. At fixed voltage, power instead follows V²/R.
Time s, min, hr Linear effect: doubling time doubles total heat.
Heat output J, Wh, kWh Total energy converted to heat during the interval.

Rearranged Forms

Because the calculator can solve for a missing variable, these algebraic rearrangements are useful when you know any three values:

Solve for Formula
Heat
RH = I² × R × t
Current
I = √(RH / (R × t))
Resistance
R = RH / (I² × t)
Time
t = RH / (I² × R)

How to Use the Calculator

  1. Enter any three known values.
  2. Select the correct units for each field.
  3. Calculate the missing variable.
  4. Interpret the result as energy dissipated as heat over the selected time period.

If you are checking whether a resistor is being overstressed, look at both the total heat energy and the instantaneous power. A resistor can accumulate a modest amount of total energy over time yet still run too hot if its wattage capability is too low for the continuous power level.

Example

For a resistor carrying 10 A with a resistance of 5 Ω for 6 hours:

P = 10² × 5 = 500
RH = 500 × 6 = 3000

The resistor dissipates 500 W continuously and produces 3000 Wh of heat energy over the full interval, which is the same as 3 kWh.

Useful Unit Conversions

Conversion Formula
Minutes or seconds to hours
tₕᵣ = tₘᵢₙ / 60 = tₛ / 3600
Wh to kWh
kWh = Wh / 1000
Wh to joules
J = Wh × 3600

The Wh-to-joule conversion is especially helpful if you need SI energy units. The optional 2× dissipation reference corresponds only to 50% loading of a nominal rating. Rating utilization and this reference do not verify suitability: use the actual part’s temperature derating, mounting, maximum voltage and pulse limits. See Tektronix RMS heating guidance, Vishay thermal-path guidance and Vishay pulse-capability guidance.

Practical Interpretation

  • Wh and kWh are energy units, so they tell you how much heat was produced in total.
  • Watts are a power unit, so they tell you how fast the resistor is heating at any moment.
  • Heat energy alone does not determine temperature. The optional temperature estimate is steady-state, using the entered thermal resistance from the chosen resistor temperature point to ambient under the actual mounting and cooling. It does not predict temperature after the entered interval.
  • Actual operating temperature depends on the part’s wattage rating, package, mounting method, airflow, heat sinking, and ambient temperature.
  • For changing current with constant resistance, use RMS current over the same interval, or sum appropriate time segments. Changing resistance requires a model for I(t)²R(t).

Common Input Mistakes

  • Using seconds or minutes in a hand calculation without first converting time to hours.
  • Confusing power with energy; watts and watt-hours are not the same thing.
  • Entering the wrong resistance value for the specific resistor being analyzed.
  • Forgetting that current has the strongest effect because it is squared in the equation.