Cash Flow Forecasting for Small Businesses

Most businesses do not die because the product failed. They die because they ran out of cash. Not slowly, with warning signs and time to react. Quickly, because the founder was looking at revenue instead of the bank balance.

Revenue is not cash. A $50K invoice that pays in 60 days does not help you make payroll next Friday. A customer who churns in March was already counted in your Q1 projections. Cash flow forecasting is about knowing, with reasonable accuracy, how much money will be in your bank account every month for the next year. It is the difference between making decisions proactively and making them in a panic.

Start With What You Know

Open a spreadsheet or whatever tool you use. The first number is your current bank balance. Real number, from your bank, today. Not what you think it should be.

Now list your fixed monthly costs. Rent, salaries, subscriptions, loan payments. These are predictable. You know what they will be next month and the month after. Write them down for each of the next twelve months. If you have a salary increase planned for July, reflect it. If your annual software renewal hits in September, put it there.

Then list your variable costs. Hosting that scales with usage. Sales commissions. Shipping costs. These change month to month, so estimate based on the last three months and your expected growth rate. Be conservative. Overestimating costs is always safer than underestimating them.

Forecast Revenue Honestly

This is where most founders get it wrong. They project revenue based on what they want to happen instead of what is likely to happen. Your cash flow forecast is not a pitch deck. Nobody is grading you on optimism. It is a survival tool, and it only works if the numbers are honest.

For recurring revenue (SaaS, subscriptions): start with your current MRR. Apply your historical churn rate monthly. Add new revenue at a rate consistent with your last three months of growth, not your best month ever. If you have been growing 8% month over month, do not project 15% because you think a new feature will change things. Use 8% or even 6% to be safe.

For non-recurring revenue (ecommerce, agency, project-based): look at your pipeline. What deals are signed? What is probable? What is wishful thinking? Only count signed contracts and high-probability deals in your base case. The wishful thinking goes in a separate optimistic scenario.

For revenue, also account for payment timing. If your average customer pays 45 days after invoice, that revenue shows up on your cash flow forecast 45 days after you earn it. Not when you send the invoice.

Build the 12-Month Projection

For each month, the formula is dead simple:

Ending Cash = Starting Cash + Cash In - Cash Out

Starting cash for month one is your current balance. Starting cash for every subsequent month is the ending cash from the previous month. Cash in is collected revenue (not earned, collected). Cash out is everything you spend.

Run this forward twelve months. You now have a line chart in your head (or on screen) that shows your cash balance trending over time. If that line hits zero, you know exactly which month it happens.

That month is your runway endpoint. And you need to take action at least three months before it arrives. Not the month before. Not when the bank balance is at $8K. Three months before, minimum.

Run Three Scenarios

A single forecast is a guess. Three scenarios give you a range. Build all three:

Base case: Current growth rate continues. No surprises, no windfalls. Churn stays consistent. You hire on your current plan.

Pessimistic case: Growth slows by 30-40%. Your biggest customer churns. That hire you planned costs 20% more than budgeted. One major unexpected expense hits. This is the scenario that tells you how bad things could realistically get.

Optimistic case: That enterprise deal closes. Growth accelerates 20% above your current rate. No major unexpected costs. This scenario tells you what you can afford to invest in if things go well.

The base case drives your decisions. The pessimistic case tells you when to worry. The optimistic case tells you where to push. If your pessimistic case shows cash running out in eight months, you need to act now. Not when the base case turns pessimistic.

When to Worry

Less than six months of runway in your base case. That is the trigger. At six months, you still have time to cut costs, close deals, or start a fundraise. At three months, your options are severely limited. Everything becomes a fire drill.

Other warning signs from your forecast: cash out growing faster than cash in for three consecutive months. Fixed costs exceeding 70% of revenue (you have no flexibility to cut). A single customer representing more than 25% of revenue (one churn event destroys your forecast).

The entire point of forecasting is to see these problems before they arrive. A founder who discovers they have four months of runway today can make changes. A founder who discovers it two months from now is scrambling.

What-If Scenarios That Matter

Beyond the three standard scenarios, run specific what-ifs for decisions you are considering:

“What if I hire two engineers next month?” Add $25K per month to your costs starting in that month. See what it does to your runway.

“What if I raise prices 20%?” Increase revenue projections by 15% (some customers will churn). See if the net effect is positive.

“What if my biggest customer leaves?” Remove that revenue from the month it would churn. See how many months of runway it costs you.

Every major spending or pricing decision should be tested against your forecast before you commit. Not after. This is not about being cautious. It is about making aggressive moves with your eyes open.

Tools for the Job

You can build a cash flow forecast in Google Sheets. Twelve columns for months, rows for each income and expense line, running totals at the bottom. It works. It is also tedious to maintain, easy to break with a misplaced formula, and running what-if scenarios means copying the entire sheet and modifying it manually.

Kartib has a forecast page with a built-in scenario builder. You set your base assumptions (current revenue, growth rate, expenses, planned hires), and it generates the 12-month projection with a cash balance chart. The scenario builder lets you toggle what-if variables and see the impact instantly. No spreadsheet gymnastics.

Update It Monthly

A forecast that was built in January and never touched again is fiction by April. At the end of each month, update actual numbers for the month that just ended and adjust projections for the remaining months. Took on a big new customer? Adjust revenue up. Lost a team member? Adjust costs down.

This takes fifteen minutes a month. It saves you from making decisions based on numbers that stopped being true three months ago.

Related Reading

Your forecast is only as good as your understanding of your burn rate. And if you want to build the P&L that feeds into your forecast, read how to build a P&L for your startup.