The formula
employer taxes = salary x employer tax rate ongoing cost = salary + employer taxes + benefits + equipment first-year cost = ongoing cost + recruiting monthly cost = ongoing cost / 12 cost multiplier = ongoing cost / salary runway cost = cash / burn − cash / (burn + monthly cost)
The multiplier is the part people quote, usually as 1.25 to 1.4 times salary. It is a reasonable sanity check and a poor decision tool, because it says nothing about whether you can carry the cost.
The last line is the one that decides hires. A €6,000-a-month person is affordable at a €35,000 burn with two years of cash and reckless at the same burn with seven months. Same cost, different answer, and only the runway calculation tells you which you are in. The wider argument is in what a hire really costs.
One caution on employer taxes: the rate varies enormously by country, from roughly 8 percent to over 40. Use your own figure rather than a benchmark, because this input moves the total more than anything else on the form.