How many ohms should a block heater have?

It depends on wattage. At 120 V, about 36 ohms corresponds to 400 W, 19.2 ohms to 750 W, 14.4 ohms to 1,000 W and 9.6 ohms to 1,500 W.

What does OL mean when testing a block heater?

OL or infinite resistance usually means the circuit is open: a broken element, disconnected cord or failed connection. Test the cord and element separately where possible.

A meter reading you can use

Block Heater Ohms & Resistance Calculator

Unplug the heater, measure resistance across its power blades, and enter the reading below. The calculator estimates the element's wattage at 120 V.

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Resistance calculator

Turn a meter reading into estimated watts

Measured resistance (Ω)Supply voltage

400 W

Estimated draw: 3.3 A at 120 V

This is a cold-resistance estimate using P = V² ÷ R. Meter-lead resistance, element temperature and electronics can affect the reading. An open circuit often displays OL; near-zero resistance suggests a short. Compare with the heater label or application listing before deciding it is healthy.

Common 120 V readings

Rated power	Approx. resistance	Current
400 W	36 Ω	3.3 A
600 W	24 Ω	5.0 A
750 W	19.2 Ω	6.25 A
1,000 W	14.4 Ω	8.3 A
1,500 W	9.6 Ω	12.5 A

Need the complete test procedure? Follow how to test a block heater safely.