A map that says 1:50,000 is making you a promise: every 1 cm on the sheet stands for 50,000 cm of real world. That is the whole formula: $d = m \cdot S$, read aloud: d equals m times S. Three letters, quickly: $m$ is your ruler measurement on the paper, in centimetres; $S$ is the big number after the 1:; and $d$ — the one you are solving for — is the real distance out in the world, in whatever unit $m$ came in. Notice $S$ has no units — it is paper-to-world, the same on both sides, which is exactly what makes it safe to multiply.

The catch is that $m \cdot S$ answers in centimetres, and nobody hikes in centimetres. Walk the chain one honest step at a time: ÷100 to reach metres, then ÷1,000 to reach kilometres — ÷100,000 end to end. On a 1:50,000 sheet that makes every centimetre half a kilometre, a fact worth keeping in your pocket.