Keep time.
A plain metronome. Set the tempo, pick where the accent lands, press start. It runs on your device's audio clock rather than a timer, so it does not drift over a long take.
Metronome
Why it doesn't drift
A metronome built on a plain timer loses time, because the browser is free to run that timer late — and over a three-minute take the error piles up. This one schedules each click against the audio clock instead, a fraction of a second ahead of when it needs to sound, so the spacing stays exact however busy the page gets.
Nothing plays until you press start, and the sound is generated on the device rather than downloaded, so there is nothing to load and it works offline once the page is open.
Half time, double time
Trap written at 140 is counted at 70 as often as not, and both are correct readings of the same song. If the click feels twice as fast as the song you are hearing, press half time — you are counting the same pulse, just at the other resolution.
Not sure of the tempo in the first place? TheBPM tapper measures it from any song you can tap along to, and the song structure calculatorturns a tempo and a bar count into an exact runtime.