The formula
frame time = 1000 ÷ FPS · refresh time = 1000 ÷ Hz
Where it comes from
A sixty hertz monitor shows a new image every 16.7 milliseconds whatever happens. If the game delivers more frames, the extras are not shown whole: without synchronisation, one arriving mid-refresh splits the screen in two, which is tearing. And if the FPS is not an exact divisor of the refresh rate, some frames are shown for one refresh and others for two, so the motion looks uneven even when the average FPS is high. That is why a hundred FPS on a sixty hertz panel can look worse than a solid sixty.
How to work it out by hand
- Work out the frame time: a thousand divided by the FPS
- Work out the refresh time: a thousand divided by the hertz
- Compare: extra FPS are not shown, missing ones get repeated
- Check whether the FPS is an exact divisor of the refresh rate
What is worth knowing
The extra frames are not entirely wasted: even unseen, they cut latency, because the one shown at each refresh is more recent. That is why competitive players push well past their panel refresh rate knowing they will not see it. The 1 % low is the figure that really describes the experience: an average of a hundred FPS with lows of forty feels far worse than a steady seventy, because what you notice is the drops and not the mean.