Rule of 40 calculator for SaaS

The Rule of 40 is a statement about trade-offs that gets used as a target. It says that above a certain scale, growth and profitability are exchangeable at par — and it says nothing whatsoever about which of the two you should be buying, what you paid for the growth you have, or whether the company is any good. On the loaded inputs the calculator returns 37.8: 50.0% growth against a -12.2% margin, below the bar.

The formula and its trade-off

score = annual revenue growth % + annual profit margin %

Both terms are annual and both are percentages, which is what makes the addition arithmetically legal. The calculator derives growth from ARR twelve months apart and margin from profit over recognised revenue for the same period, then adds them. That is the entire mechanism — there is nothing else in it.

The interesting part is what the addition asserts. Adding the two terms treats a point of growth and a point of margin as worth exactly the same, which is a strong claim and often a false one. A point of growth compounds into next year’s base; a point of margin does not. For a company at $110M of ARR growing 10.0%, that symmetry is roughly defensible. For a company at $12M growing 50.0%, it is not: the growth is worth substantially more than the equivalent margin, because it changes the size of every subsequent year.

Use the rule the way it was intended — as a sanity check on whether the burn is buying proportionate growth — and it is useful. Use it as a target and you will optimise a number that treats cutting sales headcount and accelerating revenue as identical achievements.

Which profit number to use

The formula does not specify a profit measure, and that omission does more damage than any other part of the rule. Here is one company — the loaded inputs, $8M to $12M of ARR on $9.8M of recognised revenue — scored under four measures a finance team could all defend in the same meeting:

That is 27 points of spread, and nobody has lied. The company fails on free cash flow, clears the bar on operating profit, and looks exceptional on adjusted EBITDA once share-based compensation is added back. When a company reports a Rule of 40 score without naming the measure, assume it picked the flattering one.

Use free cash flow. It is the strictest of the four and the only one that corresponds to something that happens in a bank account. Capitalised development moves engineering cost off the income statement and onto the balance sheet, which flatters operating margin without changing the payroll. Share-based compensation is a real cost that dilutes real owners. EBITDA excludes both. Free cash flow catches both, along with the working-capital effect of annual prepayments, which for a subscription business is substantial and genuinely favourable. If free cash flow is not available, use GAAP operating margin and say so.

Four companies that all score 40

This is the argument against treating the score as a ranking. Four businesses, computed through the same model, with almost nothing in common:

The first burns $2.6M a year at $9M of ARR and cannot survive a closed funding market. The last generates $31.5M of cash a year and does not need one. The rule scores them identically, and any investor would tell you they are not the same asset.

Why it breaks below $10M ARR

Below roughly $10M of ARR the rule stops carrying information, for two reasons that both come from small denominators.

The growth term becomes trivially large. A company going from $800K to $1.6M posts 100.0% growth, which sounds extraordinary and represents $800K of new ARR — roughly two enterprise contracts. Meanwhile the margin term goes to extremes: on $1.2M of revenue that company’s margin is -75.0%, giving a score of 25.

Then watch what one hire does. Add a single engineer at $180K fully loaded and the seed company’s score moves 15 points, from 25 to 10. The same hire moves the $12M company’s score by 1.9 points. A metric that swings 8 times harder on one routine hiring decision is measuring your headcount timing, not your business quality.

The perverse result is that the small company can clear the bar simply by growing off a tiny base. At 120.0% growth and a -71.0% margin the score is 49 — clears the bar — while burning $2.2M a year on $3.1M of revenue. Below $10M of ARR, track CAC payback and net revenue retention instead. The Rule of 40 starts meaning something when growth has slowed enough that the trade-off is real.

What the score cannot see

Two omissions matter more than the rest, and both concern the price of the growth rather than its size.

• CAC payback. The score sees this year’s burn and this year’s growth. It cannot see how long the customers behind that growth take to repay their acquisition cost. Two companies with identical scores, one at a nine-month payback and one at thirty months, are in completely different positions: the first is investing, the second is buying revenue on credit. Run your numbers through the LTV, CAC and payback calculator before you take any comfort from a score above 40.
• What each dollar of burn bought. The margin term tells you how much cash left; it does not tell you what came back. The burn multiple — cash burned divided by net new ARR — answers that directly and is the natural companion metric, worked through in the burn multiple guide.
• Gross margin. The rule uses revenue, not gross profit, so it is blind to the quality of that revenue. The Balanced company above, on $45M of revenue, keeps $38.3M of gross profit at an 85% margin and $24.8M at 55% — a difference of $13.5M a year, invisible to the score. A services-heavy business and a pure software business can post identical scores and be worth very different multiples.

Three ways it gets computed wrong

Annualising monthly growth by multiplying by twelve. Growth compounds, so the annual figure is (1 + monthly)¹² − 1. Multiplying understates it, and the understatement grows with the rate.

Mixing ARR growth with recognised revenue growth. These are different numbers and the score is sensitive to which you pick. On the loaded inputs, exit ARR grew 50.0%. Recognised revenue over the same period was $9.8M; if the prior year’s recognised revenue was $6.4M, revenue growth was 53.1% and the score becomes 40.9 rather than 37.8 — over the bar instead of under it, from a definitional choice. Pick one basis, apply it to both the current and prior period, and label it. ARR growth with a margin computed on recognised revenue, which is what this calculator does, is the common convention; it is defensible, but only when stated.

Ignoring gross margin entirely. Covered above, and it is the error that survives longest, because both terms of the formula are computed correctly — the score is simply answering a narrower question than the person reading it believes.

Closing the gap from here

When the score falls short, the calculator reports what each term would have to reach on its own. From the loaded inputs, at 37.8, growth would need to hit 52.2% — ARR of $12,176,000 instead of $12,000,000, so $176,000 of additional ARR — or margin would need to reach -10.0%, which is -$980,000 of profit instead of -$1,200,000, a swing of $220,000.

Those two look comparable and are not. Which is cheaper depends entirely on your CAC payback, which the rule cannot see. If payback is short, the extra ARR costs less than the profit swing and buys a permanently larger base. If payback is long, cutting is the cheaper route and the rule is telling you something true.

How to report it

Name the profit measure, name the growth basis, and publish the two terms beside the total. A score of 40 made of 60% growth and a -20% margin and a score of 40 made of 10% growth and a 30% margin are different companies, and collapsing them into one number is a loss of information the reader cannot recover. The score is a summary. Keep the inputs next to it.

Educational content, not financial advice. This tool models the assumptions you enter and excludes taxes, financing and one-off items. Check any figure that matters with a qualified accountant before you act on it.