The formula
How much rent you can afford is three numbers, not one. Your monthly gross income is your annual salary divided by 12 plus any other monthly income. Your max rent is your monthly gross times the rent rule — usually 30%. And the rent you are looking at costs a share of your monthly gross — the target rent divided by the monthly gross times 100. Whichever of the three you are solving for, the same three numbers decide it, and the salary the rent needs is (target rent ÷ rule) × 12.
Step 1 — Write down your salary and the rent rule
Start with the four numbers the calculation hangs on. In our worked example the annual gross salary is $72,000.00, the other monthly income is $0.00, the rent rule is 30%, and the target rent is $2,000.00 a month. Keep them fixed — every later step uses these four numbers, and changing one halfway through is how affordability math goes wrong.
Step 2 — Work out your monthly gross income
Your monthly gross income is your annual salary divided by 12 plus any other monthly income. At $72,000.00 a year the monthly gross is $72,000.00 ÷ 12 + $0.00 = $6,000.00. This is GROSS pay — before tax, retirement, and health deductions — so the rent that passes the test here takes a bigger bite of what actually lands in your account. The rent affordability calculator does this for you, and it finds the salary where a rent is exactly affordable.
Step 3 — Work out your max rent
Your max rent is your monthly gross times the rent rule. At $6,000.00 a month and the 30% rule the max rent is $6,000.00 × 30% = $1,800.00. That is the line: a rent at or under $1,800.00 passes the 30% test, and a rent above it does not. The 30% rule is a rule of thumb, not a law — landlords often require gross income of 3× the rent (33.33%) or 2.5× (40%) — and the rule ignores your other debts, so a rent that passes the test can still squeeze you if you carry card or loan payments.
Step 4 — Work out what the rent costs as a share of your pay
The rent’s share is the target rent divided by the monthly gross times 100. At $2,000.00 against $6,000.00 a month the share is $2,000.00 ÷ $6,000.00 × 100 = 33.33%. Under/over is the max minus the target — $1,800.00 − $2,000.00 = −$200.00 — so the $2,000.00 apartment is $200.00 over your max. The share is the number landlords actually check: at 33.33% you are at the 3× income line, and at 40% you are at the 2.5× line.
Step 5 — Work out the move-in cost and the first year
The move-in cost is the first month plus one month’s deposit — two months of rent. At $2,000.00 the move-in is 2 × $2,000.00 = $4,000.00. The first-year total is 12 months plus the deposit — 13 × $2,000.00 = $26,000.00. The move-in figure excludes utilities, parking, renters insurance, and moving costs — the real cash you need on day one is higher, so treat $4,000.00 as the floor, not the total.
Step 6 — Work out the salary the rent needs
The salary a rent needs is (target rent ÷ rule) × 12. At a $2,000.00 rent and the 30% rule the salary needed is ($2,000.00 ÷ 30%) × 12 = $80,000.00. So the question “can I afford this apartment?” has a salary attached: $80,000.00 is the line. Below it, the rent is over the 30% rule; above it, it is under — on a $2,000.00 rent at 30%, with no other income. Other monthly income lowers the salary you need: $500.00 of other income drops it to $74,000.00.
What if the rent is $1,500?
A cheaper rent changes the answer without changing the salary. At $1,500.00 against a $6,000.00 monthly gross the share is $1,500.00 ÷ $6,000.00 × 100 = 25.00% — $300.00 under the $1,800.00 max. The annual rent is $18,000.00, the move-in is $3,000.00, and the first year is $19,500.00. The salary a $1,500.00 rent needs at 30% is $60,000.00 — $20,000.00 less than the $2,000.00 rent needs.
The salary that makes a $2,000 rent exactly affordable
At an $80,000.00 salary, a $2,000.00 rent is exactly affordable at the 30% rule — the monthly gross is $6,666.67, the max rent is $2,000.00, the share is 30.00%, and the rent is exactly at the line. So the question “how much do I need to earn for this apartment?” has a number attached: $80,000.00 is the line. Below it, the rent is over; above it, it is under — at 30%, with no other income.
The rule that makes a $2,000 rent exactly affordable
At a 33.33% rule, a $2,000.00 rent is exactly affordable on $6,000.00 a month — the exact rule is 33.3333%. At the displayed 33.33% the max rent is $1,999.80 — $0.20 short of the $2,000.00 target — and 33.34% gives a max rent of $2,000.40, so 33.34% is the first rule that actually covers the rent. That is the 3× income line landlords quote: your gross income must be three times the rent, and 33.33% is the rule that says so.
The rent that gives a $5,000 move-in
At a $2,500.00 rent, the move-in cost is exactly $5,000.00 — the first month plus the deposit is 2 × $2,500.00. The first year is $32,500.00, the share is 41.67% of a $6,000.00 monthly gross, $700.00 over the $1,800.00 max, and the salary needed is $100,000.00. So the question “how much rent can I take if I have $5,000.00 to move in?” has a number attached: $2,500.00 is the line — and at $72,000.00 of salary it is $700.00 over the 30% rule.
The 25%, 30% and 40% rules — which fits you?
The 25% rule is the conservative one: on $6,000.00 a month it caps rent at $1,500.00 and needs a $96,000.00 salary for a $2,000.00 apartment — use it if you carry debt or want a cushion. The 30% rule is the standard: it caps rent at $1,800.00 and needs an $80,000.00 salary for a $2,000.00 apartment. The 40% rule is the stretch: it caps rent at $2,400.00 and needs only a $60,000.00 salary for a $2,000.00 apartment — use it only if your other costs are low, because at 40% the rent takes nearly half your gross pay and a landlord may still turn you down at the 2.5× line.