Burn Rate Calculator: Monthly Spend and Cash Runway

What does a $25,000.00 monthly burn mean?

On the defaults — $500,000 of starting cash, $350,000 of ending cash and 6 months — the burn rate is $25,000.00 a month, which means the cash falls by $25,000.00 every month — and the runway is 14.00 months, which means the $350,000 left in the bank lasts fourteen months at that spend. The burn is the cash change divided by the months: ($500,000 − $350,000) ÷ 6 = $25,000.00. A burn of $50,000.00 a month means the cash falls twice as fast, and a burn of $12,500.00 a month means it falls half as fast — the higher the burn, the shorter the runway.

Three things about the model are worth stating plainly.

First, the division is ($500,000 − $350,000) ÷ 6 = $25,000.00 a month. Second, the meaning is fourteen months of runway — the ending cash divided by the burn, $350,000 ÷ $25,000.00 = 14.00 months. And third, the burn is displayed to two decimals, half-up, and a burn that displays as $30,000.00 can sit below the line: $320,000.02 of ending cash gives 29999.9967, and a start of $529,999.98 gives the same 29999.9967 — both display as $30,000.00 but sit below $30,000 — all listed in Accuracy & limitations.

The worked example, line by line

Setup: starting cash $500,000, ending cash $350,000, 6 months.

Line 1 — the cash change. $500,000 − $350,000 = $150,000.

Line 2 — the burn. $150,000 ÷ 6 = $25,000.00 a month.

Line 3 — the runway. $350,000 ÷ $25,000.00 = 14.00 months.

Line 4 — the check. At $530,000 of starting cash the burn is $30,000.00 a month; and at $529,999 it is $29,999.83 — below $30,000, though it displays as $30,000.00.

Every branch, checked

Months 3: starting cash $500,000.00; ending cash $350,000.00; burn $50,000.00/month; runway 7.00 months — Over 3 months the same $150,000 of cash change burns at $50,000.00 a month, and the runway is only 7.00 months.

Months 12: starting cash $500,000.00; ending cash $350,000.00; burn $12,500.00/month; runway 28.00 months — Over 12 months the burn falls to $12,500.00 a month, and the runway stretches to 28.00 months.

Months 5: starting cash $500,000.00; ending cash $350,000.00; burn $30,000.00/month; runway 11.67 months — Over 5 months the burn is $30,000.00 a month, and the runway is 11.67 months.

Start $600,000: starting cash $600,000.00; ending cash $350,000.00; burn $41,666.67/month; runway 8.40 months — Starting with $600,000 lifts the burn to $41,666.67 a month (unrounded 41666.6667), and the runway shortens to 8.40 months.

Start $400,000: starting cash $400,000.00; ending cash $350,000.00; burn $8,333.33/month; runway 42.00 months — Starting with $400,000 drops the burn to $8,333.33 a month, and the runway stretches to 42.00 months.

End $200,000: starting cash $500,000.00; ending cash $200,000.00; burn $50,000.00/month; runway 4.00 months — Ending with $200,000 lifts the burn to $50,000.00 a month, and the runway is only 4.00 months.

End $450,000: starting cash $500,000.00; ending cash $450,000.00; burn $8,333.33/month; runway 54.00 months — Ending with $450,000 drops the burn to $8,333.33 a month, and the runway stretches to 54.00 months.

End $300,000: starting cash $500,000.00; ending cash $300,000.00; burn $33,333.33/month; runway 9.00 months — Ending with $300,000 lifts the burn to $33,333.33 a month, and the runway is 9.00 months.

The reverses, worked backwards

Smallest starting cash for the burn to REACH $30,000.00/month: At the displayed $530,000 the figures are starting cash $530,000.00; ending cash $350,000.00; burn $30,000.00/month; runway 11.67 months. At $530,000 the burn is exactly $30,000.00 a month — on the line. At $529,999 the burn is $29,999.83 a month (unrounded 29999.8333) — it displays as $30,000.00 but is below the target — so $530,000 is the smallest starting cash for the burn to reach $30,000.00 a month.

Largest ending cash for the burn to STAY at or above $30,000.00/month: At the displayed $320,000 the figures are starting cash $500,000.00; ending cash $320,000.00; burn $30,000.00/month; runway 10.67 months. At $320,000 the burn is exactly $30,000.00 a month — on the line. At $320,001 the burn is $29,999.83 a month (unrounded 29999.8333) — it displays as $30,000.00 but is below the target — so $320,000 is the largest ending cash for the burn to stay at or above $30,000.00 a month.

Largest months for the burn to STAY at or above $50,000.00/month: At the displayed 3 months the figures are starting cash $500,000.00; ending cash $350,000.00; burn $50,000.00/month; runway 7.00 months. At 3 months the burn is exactly $50,000.00 a month — on the line. At 4 months the burn is $37,500.00 a month — below the target — so 3 months is the largest months for the burn to stay at or above $50,000.00 a month.

Smallest months for the burn to FALL to $25,000.00/month or below: At the displayed 6 months the figures are starting cash $500,000.00; ending cash $350,000.00; burn $25,000.00/month; runway 14.00 months. At 6 months the burn is exactly $25,000.00 a month — on the line. At 5 months the burn is $30,000.00 a month — above the target — so 6 months is the smallest months for the burn to fall to $25,000.00 a month or below.

Smallest starting cash for the burn to REACH $20,000.00/month: At the displayed $470,000 the figures are starting cash $470,000.00; ending cash $350,000.00; burn $20,000.00/month; runway 17.50 months. At $470,000 the burn is exactly $20,000.00 a month — on the line. At $469,999 the burn is $19,999.83 a month (unrounded 19999.8333) — it displays as $20,000.00 but is below the target — so $470,000 is the smallest starting cash for the burn to reach $20,000.00 a month.

Largest ending cash for the burn to STAY at or above $20,000.00/month: At the displayed $380,000 the figures are starting cash $500,000.00; ending cash $380,000.00; burn $20,000.00/month; runway 19.00 months. At $380,000 the burn is exactly $20,000.00 a month — on the line. At $380,001 the burn is $19,999.83 a month (unrounded 19999.8333) — it displays as $20,000.00 but is below the target — so $380,000 is the largest ending cash for the burn to stay at or above $20,000.00 a month.

A warning about the neighbours: every dollar neighbour above sits one dollar from the star, and every neighbour burn displays as the target when rounded to two decimals, yet sits below it. The burn is linear in starting cash and one over months, so every reverse has an exact closed form: the starting cash needed is the target times the months plus the ending cash, and the ending cash allowed is the starting cash minus the target times the months. The page prints the unrounded burns in full so the rounding does not mislead you: 29999.8333 and 19999.8333.

What the defaults assume

The defaults are starting cash $500,000, ending cash $350,000 and 6 months — starting cash $500,000.00; ending cash $350,000.00; burn $25,000.00/month; runway 14.00 months. With Months 3, starting cash $500,000.00; ending cash $350,000.00; burn $50,000.00/month; runway 7.00 months. With Months 12, starting cash $500,000.00; ending cash $350,000.00; burn $12,500.00/month; runway 28.00 months. With Months 5, starting cash $500,000.00; ending cash $350,000.00; burn $30,000.00/month; runway 11.67 months. With Start $600,000, starting cash $600,000.00; ending cash $350,000.00; burn $41,666.67/month; runway 8.40 months (unrounded 41666.6667). With Start $400,000, starting cash $400,000.00; ending cash $350,000.00; burn $8,333.33/month; runway 42.00 months. With End $200,000, starting cash $500,000.00; ending cash $200,000.00; burn $50,000.00/month; runway 4.00 months. With End $450,000, starting cash $500,000.00; ending cash $450,000.00; burn $8,333.33/month; runway 54.00 months. With End $300,000, starting cash $500,000.00; ending cash $300,000.00; burn $33,333.33/month; runway 9.00 months. With Months 0, starting cash $500,000.00; ending cash $350,000.00; burn undefined — cash change ÷ zero — cash change ÷ zero has no answer. With Ending cash $400,000 (above the start), starting cash $350,000.00; ending cash $400,000.00; burn −$8,333.33/month (unrounded −8333.3333); runway not burning cash (unrounded −8333.3333) — the business is adding cash, not spending it. With Ending cash $320,000.02 (rounding trap), starting cash $500,000.00; ending cash $320,000.02; burn $30,000.00/month (unrounded 29999.9967); runway 10.67 months (unrounded 29999.9967) — displays $30,000.00 but is below the $30,000 target. With Starting cash = ending cash $350,000, starting cash $350,000.00; ending cash $350,000.00; burn $0.00/month; runway not burning cash — no burn at all. With Starting cash $10,000,000, ending cash $7,500,000, 10 months, starting cash $10,000,000.00; ending cash $7,500,000.00; burn $250,000.00/month; runway 30.00 months. With Ending cash $0 (out of cash), starting cash $500,000.00; ending cash $0.00; burn $83,333.33/month (unrounded 83333.3333); runway 0.00 months (unrounded 83333.3333) — out of cash. With 6.5 months (fractional months), starting cash $500,000.00; ending cash $350,000.00; burn $23,076.92/month (unrounded 23076.9231); runway 15.17 months (unrounded 23076.9231). With 1 month, starting cash $500,000.00; ending cash $350,000.00; burn $150,000.00/month; runway 2.33 months. With Starting cash $100,000, ending cash $250,000 (profit), starting cash $100,000.00; ending cash $250,000.00; burn −$30,000.00/month; runway not burning cash — adding $30,000 a month. With Fractional inputs, starting cash $437,654.32; ending cash $123,456.78; burn $52,366.26/month (unrounded 52366.2567); runway 2.36 months (unrounded 52366.2567). With Starting cash $529,999.98 (start-side trap), starting cash $529,999.98; ending cash $350,000.00; burn $30,000.00/month (unrounded 29999.9967); runway 11.67 months (unrounded 29999.9967) — displays $30,000.00 but is below the $30,000 target.

A note on the rounding: the burn is displayed to two decimals, half-up. At $320,000.02 of ending cash the unrounded burn is 29999.9967 — it displays as $30,000.00 but is below the $30,000 target. At $529,999.98 of starting cash the unrounded burn is 29999.9967 — it displays as $30,000.00 but is below the $30,000 target. At $529,999 of starting cash the unrounded burn is 29999.8333 — it displays as $30,000.00 but is below the $30,000 target. At $320,001 of ending cash the unrounded burn is 29999.8333 — it displays as $30,000.00 but is below the $30,000 target. At $469,999 of starting cash the unrounded burn is 19999.8333 — it displays as $20,000.00 but is below the $20,000 target. At $380,001 of ending cash the unrounded burn is 19999.8333 — it displays as $20,000.00 but is below the $20,000 target. With the fractional inputs the unrounded burn is 52366.2567 — it displays as $52,366.26. The page prints the unrounded burns so the rounding does not mislead you.

Output · formula · meaning

Who uses this

• The founder checking the runway: they learn $500,000 of starting cash on $350,000 of ending cash over 6 months burns $25,000.00 a month — a 14.00-month runway — and they check the $530,000 reverse to see the starting cash that pushes the burn to $30,000.00 a month.
• The investor screening a startup: they learn a burn of $50,000.00 a month leaves only 7.00 months of runway — and they check the $320,000 reverse to see the ending cash that holds the burn at or above $30,000.00 a month.
• The finance lead planning the next raise: they learn the burn falls to $12,500.00 a month over 12 months — and they check the 6-month reverse to see the shortest period that keeps the burn at or below $25,000.00 a month.
• The student learning startup finance: they learn the burn is (starting cash − ending cash) ÷ months — and they check the $470,000 reverse to see the starting cash that pushes the burn to $20,000.00 a month.

Accuracy & limitations

The math is first-principles: the burn rate is (starting cash − ending cash) divided by months — all in dollars, the burn displayed to two decimals half-up, and the runway is ending cash divided by the UNROUNDED burn. At 0 months the burn is UNDEFINED (cash change ÷ zero — division by zero) — the calculator shows “undefined” with a note, never a number, never hidden. At $400,000 of ending cash, above the $350,000 start, the burn is −$8,333.33 — a negative burn, the business is ADDING cash, not spending it — and the runway is “not burning cash”, never a number — the calculator shows the −$8,333.33 figure with a note, never hides it. At $350,000 of ending cash, equal to the start, the burn is $0.00 — no burn at all — and the runway is “not burning cash”, never a number — the calculator shows the $0.00 figure with a note, never hides it. At $100,000 of starting cash and $250,000 of ending cash the burn is −$30,000.00 — adding $30,000 a month — and the runway is “not burning cash”, never a number. At $0 of ending cash the burn is $83,333.33 and the runway is 0.00 months — out of cash. What the model cannot do is predict the future: the burn is backward-looking, it assumes the spend stays flat, and it says nothing about revenue coming in, fundraising, or one-off costs — a single big payment in one month makes the burn look worse than the steady spend. Starting and ending cash must come from the same bank accounts, in the same currency, or the burn is wrong before it starts. Use it to see how fast the cash is going and how long it lasts — then check the revenue, the hiring plan and the bank balance before you decide when to raise.

Frequently asked questions