Calculate startup runway with income and one-off costs

When this helps

You are deciding whether to hire or pay for a one-off project. The current bank balance is not enough to answer: recurring receipts, monthly spending and the upfront payment all affect the time available. Put those assumptions in one calculation note.

How to calculate it

1. Subtract the one-off cash expense from the starting cash balance.
2. Subtract monthly cash receipts from monthly cash spending to find net cash burn.
3. Divide the remaining cash by net monthly burn to estimate months of runway.

Inputs and formulas

Copy these lines into Sumico or use the interactive example below. Decimal values in expressions use a dot.

# Cash runway
cash = $84000
income = $6000
spend = $13000
once = $14000

available = cash - once
burn = spend - income
Months: available / burn

Read the result

After the one-off payment, $70,000 remains. Net monthly burn is $7,000, so runway is 10 months. Omitting the one-off expense would incorrectly show 12 months at the same burn.

The calculation in Sumico

Change an assumption

Change spend = $13000 to spend = $16000. Net monthly burn rises to $10,000 and runway falls to 7 months. This shows the cash effect of an additional recurring expense, assuming receipts stay unchanged.

Assumptions and limits

This is a constant-burn estimate, not a cash-flow forecast. Receipts mean cash actually received, not invoices or booked revenue. It excludes timing differences, growth, financing and future one-off expenses. If burn is zero or negative, a finite depletion runway does not apply under these assumptions; use a month-by-month model for uneven cash flows.

A common question

Should I divide cash by all monthly spending?

Use net cash burn when you have recurring cash receipts: spending minus receipts. Dividing by total spending ignores the cash coming in and produces a different estimate.