A salary offer quotes gross pay: the amount before income tax, social contributions and any deduction you pay yourself. What you can spend is net pay, and the distance between the two is the first thing a new job teaches you.
The gap is not a fixed percentage. It depends on how your country taxes income, whether contributions are capped, whether you have dependants, and whether you pay into a pension yourself. A single effective rate, which is total tax divided by total income, reproduces the outcome closely enough to plan with. The Salary Calculator turns a gross salary and an effective rate into a monthly take-home figure.
Then look at everything that is not salary. An employer pension contribution is money paid on your behalf and is usually worth more than the same amount in gross pay, because no contribution is deducted from it. Health cover, a car allowance, a commuting subsidy, extra leave and flexible working all have a cash equivalent. Two offers with the same base salary can differ by a tenth of their total value once these are counted, which is what the Job Offer Comparison Calculator works out.
Finally, compare the two salaries where you would actually live. A higher salary in an expensive city can leave you with less at the end of the month than a lower one somewhere cheaper, which is a comparison of purchasing power rather than of income. Run both numbers through the Cost of Living Calculator before treating a pay rise as a pay rise.
One habit makes all of this easier: keep your budget in net terms. Rent, loans, subscriptions and food are paid from take-home pay, so a budget built on gross income is wrong from its first line. The Budget Calculator starts from net income for exactly that reason.