Support
RPMCalc — Engine RPM From Sound
Questions about using RPMCalc? Start with the FAQ below. If you don't find your answer, email us directly — we read every message.
Frequently Asked Questions
Why does RPMCalc show "Listening…" but never a number?
RPMCalc only accepts a reading once it's confident it's hearing an actual engine tone, not background noise. Get the phone as close to the exhaust or engine as safely possible, reduce wind noise if you're outdoors, and make sure the cylinder count and 2-stroke/4-stroke setting match the engine you're listening to — those settings tell RPMCalc which frequency range to listen in.
The RPM number looks way too high or too low. What's wrong?
This almost always means the cylinder count or stroke-cycle setting doesn't match the engine. RPMCalc derives RPM directly from those two settings plus the sound's frequency, so a wrong cylinder count will scale the result incorrectly. Double check both before listening. Also check for other engines, fans, or machinery running nearby — a second sound source in range can be picked up instead.
How do I use RPMCalc by myself, without someone watching the phone?
Use Auto-Freeze. Set your desired delay (5–30 seconds) before tapping Start Listening, then prop the phone securely near the exhaust and go rev the engine. RPMCalc will automatically freeze the reading after the delay elapses, so it's waiting for you when you walk back — no need to watch the screen while revving. You can also tap Freeze at any moment to hold the current reading manually.
RPMCalc asked for microphone access — is my audio recorded?
No. RPMCalc analyzes sound in real time to calculate a frequency and never records, stores, or transmits any audio. See our Copyright & Privacy page for full details.
I denied microphone access by accident. How do I turn it back on?
Go to iOS Settings > Privacy & Security > Microphone and enable access for RPMCalc. If RPMCalc's Start Listening button shows an "Open Settings" prompt, tapping it will take you straight there.
What cylinder counts and engine types does RPMCalc support?
Any cylinder count from 1 to 12, in either 2-stroke or 4-stroke configuration. That covers single-cylinder small engines (mowers, chainsaws, generators), motorcycles, most car and truck engines, and larger multi-cylinder equipment.
Does RPMCalc work as a certified tachometer for tuning or inspections?
No. RPMCalc is a reference tool that estimates RPM acoustically — it is not a certified or calibrated instrument. For safety-critical decisions like redline limits, tuning, or inspections, always use your equipment manufacturer's official instrumentation and guidance.
Does RPMCalc need an internet connection?
No. Every part of RPMCalc, including all audio analysis, runs entirely on your device. It works fully offline.