Why does the bottom number matter?

Because it says what note the beats are. A bar of 6/8 is six eighth notes, which is three quarter notes, and tempo is quoted in quarter notes almost everywhere. A calculator that treats the top number as beats reports a piece in 6/8 at twice its real length, and that is the commonest error in this category.

What tempo does it assume?

That the number you give is quarter notes a minute, which is what a metronome marking means and what every DAW displays. If your score marks a dotted quarter at 60 in 6/8, that is 90 quarter notes a minute, so enter 90.

Will this match my recording?

Only if the tempo is constant. Anything played rather than programmed drifts, and a piece with a ritardando or a tempo change will come out different. This is the arithmetic for a fixed tempo, which is exactly right for a click track and approximate for anything else.

Song Length Calculator

Bars to minutes, and back.

Runs in your browserNothing you enter is uploaded, and it keeps working with the network off.

Convert

bars to timetime to bars

Tempo

In quarter notes, which is how tempo is quoted.

Beats in a bar

The top of the time signature.

Counted in

The bottom of the time signature.

Bars

Used when converting bars to time.

Length in seconds

Used when converting time to bars.

Bars to minutes, and back.

How to use it

Pick the direction. Bars into a running time, or the reverse.
Give the tempo and time signature. Both numbers matter, not just the top.
Read the length. Or the bar count.

When you would use this

Bars times the length of a bar. The only difficulty is what a bar is. A time signature has two numbers and most calculators use one. The top says how many beats are in a bar and the bottom says what kind of note those beats are, and the bottom is the one that gets dropped. A bar of 6/8 is six eighth notes. Six eighth notes is three quarter notes. Tempo is quoted in quarter notes, so a bar of 6/8 at 120 lasts a second and a half, not three seconds, and a tool ignoring the bottom number reports every piece in compound time at double. The same applies upward. A bar of 2/2, cut common, is two half notes, which is four quarter notes, so it lasts exactly as long as a bar of 4/4 at the same tempo despite having half as many beats in it. Both directions are here. Bars to time is what you want when laying out an arrangement. Time to bars is what you want when a piece of music has to fit a video of a known length. What the arithmetic assumes is a constant tempo, and it says so. Anything actually played drifts, and a piece with a ritardando or a tempo change will not match. For a click track it is exact; for a recording it is a good estimate of the middle.

Questions

Why does the bottom number matter?: Because it says what note the beats are. A bar of 6/8 is six eighth notes, which is three quarter notes, and tempo is quoted in quarter notes almost everywhere. A calculator that treats the top number as beats reports a piece in 6/8 at twice its real length, and that is the commonest error in this category.
What tempo does it assume?: That the number you give is quarter notes a minute, which is what a metronome marking means and what every DAW displays. If your score marks a dotted quarter at 60 in 6/8, that is 90 quarter notes a minute, so enter 90.
Will this match my recording?: Only if the tempo is constant. Anything played rather than programmed drifts, and a piece with a ritardando or a tempo change will come out different. This is the arithmetic for a fixed tempo, which is exactly right for a click track and approximate for anything else.