Cash Flow Calculator — Project Monthly Balance & Runway
Project your running cash balance month by month from a starting balance and monthly inflows and outflows, with net flow, ending balance, and runway. Free.
About Cash Flow Calculator
A cash flow calculator is a tool that projects your cash balance forward month by month, so you can see how much money you will have on hand at the end of each period. Starting from your current balance, it applies a constant monthly inflow (money coming in) and a constant monthly outflow (money going out) to compute the net monthly cash flow and roll the balance forward. If you are spending more than you take in, it also estimates your runway — the number of months until the balance reaches zero. It is a simple constant-flow estimate, not financial advice, and runs entirely in your browser.
Use cases
- Estimate a startup’s runway. For an early-stage business burning more cash than it earns, runway is the single most important number: how long until the money runs out. Entering your bank balance, monthly revenue, and monthly spend gives a clear month count, so you can see how much time you have to reach profitability, cut costs, or raise more before the balance hits zero.
- See the effect of cutting costs or raising revenue. Runway responds sharply to changes in monthly flow. By lowering the outflow or raising the inflow you can watch the ending balance and runway improve, turning vague intentions like ‘spend less’ into a concrete extension of how many months you can keep operating. It makes the payoff of each cost cut or new revenue stream easy to compare.
- Plan for a large upcoming balance drop. If you expect the balance to fall steadily, projecting it forward shows the month it would drop below a threshold you care about — a minimum operating buffer, for example. Seeing that month in advance gives you time to act, whether that means arranging finance, delaying discretionary spending, or accelerating collections before the cushion runs thin.
- Track a household or personal cash cushion. The same month-by-month projection works for personal finances. Enter your savings balance, monthly income, and monthly expenses to see whether the cushion grows or shrinks over the year, and how many months it would last if income stopped. It is a simple, private way to picture your financial trajectory without building a spreadsheet.
- Test a hiring or expansion decision. Adding a salary or a new lease raises your monthly outflow, which shortens runway. By increasing the outflow to reflect the new commitment you can see how many months of runway it costs and whether the balance still stays above zero for as long as you need, before you make the hire or sign the lease.
How it works
- Enter your starting balance. Type the cash you have on hand right now — the balance the projection begins from.
- Enter monthly inflow. Add the money you expect to come in each month, such as revenue, salary, or other income.
- Enter monthly outflow. Add your total monthly spending — costs, expenses, and any recurring payments.
- Choose how many months to project. Set the horizon; the tool rolls the balance forward one month at a time across that period.
- Read the results. See the net monthly cash flow, the projected ending balance, a month-by-month balance list, and the runway if you are spending more than you earn.
Examples
Input: Start $50,000; in $10,000/mo; out $15,000/mo
Output: Net −$5,000/mo; runway 10 months
Burning $5,000 a month against a $50,000 balance empties it in 10 months.
Input: Start $20,000; in $8,000/mo; out $6,000/mo
Output: Net +$2,000/mo; no runway limit
Positive net flow means the balance grows, so there is no month it hits zero.
Input: Start $30,000; in $5,000/mo; out $8,000/mo, 12-month view
Output: Balance reaches $0 in month 10
At −$3,000/mo the projection crosses zero before the 12-month horizon ends.
Frequently asked questions
What is net monthly cash flow?
It is monthly inflow minus monthly outflow. A positive number means your balance grows each month; a negative number means it shrinks. It is the single figure that drives whether you build a cushion or burn through one.
What is runway?
Runway is the number of months until your cash balance reaches zero at the current burn rate. It is calculated only when you are spending more than you take in; with positive cash flow there is no runway limit, because the balance keeps rising.
How is the balance projected?
The tool starts from your opening balance and adds the net monthly cash flow once per month across the horizon you choose, listing the balance at the end of each month. It assumes the same inflow and outflow every month.
What does this calculator NOT model?
It uses constant monthly flows, so it does not handle seasonality, one-off events like a big invoice or tax bill, the timing of receivables and payables, interest, or financing rounds. Real cash flow is lumpier; treat this as a smooth-line estimate.
Can I use it for personal budgeting?
Yes. Use your savings as the starting balance, your income as the inflow, and your expenses as the outflow. The same projection shows whether your cushion grows or shrinks over the months ahead and how long it would last.
Why does it say no runway?
When your monthly inflow is greater than or equal to your outflow, the balance never falls to zero, so there is no finite runway. The tool reports steady growth instead of a countdown.
Is this financial advice?
No. It is an educational estimate based on the constant figures you enter. For real decisions — funding, budgeting, or planning around uneven cash — consult a qualified accountant or financial professional.
Pro tips
- Use conservative, slightly pessimistic inflow figures so the runway you plan around carries a safety margin.
- Fold irregular costs into the monthly outflow by averaging them, and remember the projection still smooths their timing.
- Re-run whenever revenue or spending changes materially — runway shifts quickly with small monthly changes.
- Keep a minimum buffer in mind; you usually cannot pay bills well before the balance actually reaches zero.
- Pair this with a break-even or amortizing-loan estimate to see the full picture before committing to new spending.
Reviewed by Ahsan Mahmood · Last updated 2026-07-08 · Part of ZTools.
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