Testing & Measurement calculator
Insulation Resistance Calculator
This insulation resistance calculator is an honest megger-reading screen for three specific jobs: correcting a spot reading to one reference temperature, comparing two readings at the same reference temperature, and checking a rotating machine against the common IEEE 43 style (kV + 1) MOhm rule-of-thumb at 40C. Example: a 50 MΩ reading at 25C corrects to about 17.7 MΩ at 40C with the 2:1 per 10C approximation. The engine is appropriate for quick field normalization and trend review. It does not calculate PI, DAR, hi-pot withstand, tan delta, or cable acceptance tables.
Updated July 10, 2026
A 50 MΩ megger reading at 25°C corrects to about 17.7 MΩ at 40°C using the common 2:1 per 10°C rule-of-thumb.
R40 ≈ Rmeas × 0.5^((40-25)/10) = 50 × 0.3536 = 17.7 MΩ
Choose temperature correction, rotating-machine screen, or corrected trend mode below for your megger reading
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Field kit
Test gear for insulation checks
Use the correction result as documentation support, then compare instruments built for insulation-resistance testing.
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How to Use
How to use the insulation resistance calculator
Use this page when the immediate question is how a megger reading changes with temperature or how two readings compare once they are corrected to the same reference.
1. Choose the mode that matches the task
- Temperature Correction corrects one measured insulation-resistance reading to a chosen reference temperature.
- Rotating Machine Rule-of-Thumb Screen corrects the reading to 40C and compares it with the common (kV + 1) MOhm minimum screen used for motors and generators.
- Trend Comparison at One Reference Temperature corrects the current and previous readings to the same reference so the comparison is meaningful.
2. Enter the measured reading and the test temperature
- Measured Insulation Resistance (MOhm) is the megger reading from the field test.
- Current Test Temperature (C) should match the winding, conductor, or ambient temperature logged with that reading.
- Reference Temperature (C) is editable in the correction and trend modes; use 40C unless your procedure calls for another reference.
- Applied DC Test Voltage records the actual megger test voltage used during the reading.
3. Use the rotating-machine screen only where it belongs
- The Rated Equipment Voltage field is only used in the rotating-machine screen.
- The page then corrects the reading to 40C and compares it with the simple screen (kV rated + 1) MOhm.
- That screen is helpful for quick motor or generator review, but it is not a universal pass/fail rule for transformers, cables, feeders, or building wiring.
4. Understand the formula behind the correction
The calculator uses the common field approximation:
Rref ≈ Rmeasured x 0.5^((Tref - Tmeasured) / 10)
That means insulation resistance is screened as roughly halving for each 10C increase in temperature and roughly doubling for each 10C decrease. This is useful for quick normalization, but formal acceptance work should still follow the adopted standard and the equipment manufacturer.
5. Know what this page does not do
- It does not calculate polarization index (PI) or dielectric absorption ratio (DAR).
- It does not replace cable hi-pot, VLF, or factory acceptance procedures.
- It does not create a universal minimum table for all equipment classes.
6. Use related tools when you need another workflow
- Use the Cable Testing Calculator when the work shifts to cable-focused diagnostics.
- Use the Insulation Resistance Temperature Correction Worksheet when the next step is recording the corrected reading, test temperature, equipment ID, and baseline trend.
- Use the Grounding Resistance Calculator when the concern is grounding system performance instead of insulation condition.
- Use the Arc Flash Calculator separately for energized-work safety studies.
Common Applications
More applications. Open to review 2 additional use cases.
Frequently Asked Questions
How do I correct insulation resistance for temperature?
Use the measured resistance, the test temperature, and the reference temperature. This calculator applies the common halving-per-10C approximation: Rref ≈ Rmeasured x 0.5^((Tref - Tmeasured) / 10). That lets you compare readings taken in different seasons or under different operating conditions.
What does the (kV + 1) MOhm screen mean?
It is a common rotating-machine rule-of-thumb used as a quick screen after the reading is corrected to 40C. For example, a 480 V motor screens against about 1.48 MOhm. It is useful for fast review, but final acceptance still depends on the adopted standard, manufacturer guidance, and trend history.
Does this calculator replace PI, DAR, or a full acceptance test?
No. This page handles temperature correction, a rotating-machine screen, and corrected trend comparison only. Timed tests such as PI and DAR, as well as cable hi-pot or commissioning acceptance work, still need their own procedures and instruments.
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