Cash flow runway calculator: turn burn rate and cash balance into months of runway so you know when your startup needs funding. Free, runs in your browser.
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Cash flow runway calculator: turn burn rate and cash balance into months of runway so you know when your startup needs funding. Free, runs in your browser.
5% monthly expense growth cut the runway from 50 months to 12.
Transparent methodology
How this calculator works
Reviewed July 2026 · BoringToolsKit Editorial Team
Formula
Starting cash is updated month by month by adding modeled revenue and subtracting modeled expenses; each may compound by its entered monthly change. Runway is the first month ending cash reaches zero or below.
Worked example
$10,000 starting cash with $2,000 monthly revenue and $3,000 monthly expenses reaches zero at month 10 when both remain constant.
BoringToolsKit. “Cash-Flow Runway Calculator.” boringtoolskit.com/cash-flow-runway-calculator/ (reviewed July 2026). Free to reference in articles, syllabi, and answer posts with a link.
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Worked example: the arithmetic behind an 8-month runway
Take $12,000 in cash, $3,000 monthly revenue, $4,500 monthly expenses, no growth, 24-month horizon. The net burn is $1,500 per month, so each row subtracts 1,500: month 1 ends at $10,500, month 2 at $9,000, month 3 at $7,500, and so on. Cash hits exactly $0 in month 8 ($12,000 minus 8 x $1,500), which the loop detects with the cash <= 0 test, so the reported runway is Month 8 and the table stops there even though 24 months were requested.
The other outputs follow from the same loop. Lowest cash is $0, the value at the depletion month itself, so the metric can dip to zero or below rather than showing the last positive balance. Ending cash is also $0. Operating break-even is Not reached, because the break-even flag only records the first month where revenue >= expenses, and $3,000 never reaches $4,500 here. To push the runway out, the model responds linearly to flat inputs: raising revenue to $4,500 makes every month net zero, the cash line stays flat at $12,000 for all 24 rows, and the headline flips to Beyond 24 months.
Compounding expenses cut runway faster than intuition suggests
Growth rates apply multiplicatively each month, which punishes expense growth hard. Start with $50,000 cash, $10,000 revenue, $11,000 expenses, all flat: the $1,000 monthly deficit gives a 50-month runway. Now add just 5% monthly expense growth with revenue flat. Expenses hit $11,550 in month 2, $12,155 in month 3, and the deficit widens every row. Cash falls to about $11,639 at month 10, $3,722 at month 11, and goes negative in month 12, when expenses reach $11000 x 1.05^11 = $18,856 against $10,000 revenue. A 5% monthly drift in costs cuts the runway from 50 months to 12.
Two structural limits matter when reading results. First, revenue and expense growth are fixed percentages, so the model cannot express hiring plans, seasonal revenue, or a funding round; a $2,000,000 investment in month 6 has no input field. Second, the depletion month is exact only because the loop stops at the first non-positive cash balance, and the headline does not distinguish ending at $0.00 from going deeply negative, so check the final table row's ending cash figure before quoting the runway to anyone.
