Every SaaS dashboard shows MRR, churn and CAC payback. Three metrics that explain far more about where a business is actually going almost never appear, because each one needs a definition agreed before it means anything: net revenue retention, the Rule of 40, and the magic number.
Here is each one, and the specific way it gets quietly corrupted.
1. Net revenue retention — the denominator is the whole metric
NRR = revenue from the starting cohort today ÷ revenue from the same cohort at period start
Above 100% means the existing base grows without a single new customer. Below 100% means you must win new business just to stand still.
Two variants, and you should report both:
| NRR | GRR | |
|---|---|---|
| Expansion included | yes | no |
| Can exceed 100% | yes | no |
| Answers | is the base compounding? | how much leaks out? |
A company at 115% NRR and 82% GRR is growing its base while losing nearly a fifth of it every year. That dependency on continued upsell is invisible if you only quote NRR.
The corruption is almost always the cohort. New customers signed during the period must be excluded from both numerator and denominator. A customer who signed in March and bought more in June is a new logo expanding, not retention.
The other two inflators: a list price rise applied across the base (real revenue, but it does not recur next year), and non-recurring revenue sitting in the starting base — professional services, onboarding fees, usage overage. Compute on recurring revenue only.
Implementation, if you have customer id / plan / MRR per month:
cohort = mrr[mrr.month == t0].query("mrr > 0")
denom = cohort.mrr.sum()
num = mrr[(mrr.month == t0 + 12) & mrr.id.isin(cohort.id)].mrr.sum()
nrr = num / denom
Do this for twelve monthly cohorts and average. A single window picks up whatever renewal cycle happened to land inside it.
2. Rule of 40 — the margin definition is the argument
score = revenue growth rate (%) + profit margin (%)
30% growth with a 10% margin passes. So does 10% growth with a 30% margin. That is the point: it encodes a trade-off rather than demanding both.
The ambiguity is "margin". Free cash flow margin is the original public-company definition and the hardest to flatter. EBITDA margin is the most common. Operating margin is stricter. Whichever you pick, keep it — switching definitions between periods moves the score by more than most operational improvements do.
What the score does not tell you:
- It punishes deliberate investment. Hiring a sales team ahead of demand scores badly this year and well next year.
- It treats all growth as equal. 25% growth at 110% NRR is a compounding base; 25% at 75% NRR is a treadmill. Same score.
- Small numbers distort it. Below roughly $1m ARR the percentages are noise.
- It ignores capital efficiency. Growth bought with $3 of equity per $1 of ARR passes the rule and destroys returns.
Use it as one of four signals, alongside NRR/GRR, CAC payback and runway. A company can pass the Rule of 40 and be nine months from insolvency if its cash conversion is poor.
3. Magic number — the lag is doing all the work
magic = (ARR this quarter − ARR last quarter) ÷ S&M spend last quarter
Above 0.75 is considered efficient enough to increase spend. Below 0.5 means acquisition is not paying back fast enough to justify more.
Three details decide whether the number means anything:
- Net new ARR includes churn. Using gross new business makes any retention problem disappear — exactly the failure the metric exists to catch.
- Full S&M cost. Salaries, commissions, ads, tools, agencies. Excluding salaries inflates it, often by 2x.
- The lag. One quarter is convention. If your sales cycle is nine months, one quarter credits revenue to the wrong spend and understates efficiency badly.
What it hides: contract length (a 0.8 looks identical whether you bill monthly or prepay three years), gross margin (it compares ARR to spend, not gross profit to spend), and lifetime entirely. Enterprise at 0.6 with an 8-month payback can be better business than SMB at 0.9 with a 22-month payback.
Use the magic number as the budget dial and CAC payback as the viability floor. When they disagree, the payback is usually right and the magic number is measuring the wrong thing.
The common thread
All three fail the same way: the arithmetic is trivial and the definition is not. NRR needs a cohort rule, the Rule of 40 needs a margin rule, the magic number needs a lag rule. Decide those three once, write them down, and the numbers stop moving between meetings.
Longer write-ups with worked examples and segment benchmarks:
- Net revenue retention — NRR vs GRR, cohort rules, the three inflators
- The Rule of 40 — which margin, score bands, where it misleads
- SaaS magic number — the lag problem, what it hides
I also built free no-signup calculators for churn, runway and CAC payback, each showing its formula and derivation.
Top comments (1)
The line that ties it together is "the arithmetic is trivial and the definition is not." NRR is the best example: one number that quietly encodes three judgments — who counts as the cohort, what counts as the same revenue, and which inflators you're pretending not to see. Two companies can both report "NRR 112%" and be describing completely different businesses.
One practical habit this argues for: never ship the number without the definition. A dashboard cell that reads "NRR 112% (fixed 12-mo cohort, new logos excluded, price rises included)" is worth ten dashboards of bare percentages. Your corruption modes are exactly the checklist for that — they read like a spec for what the tooltip under each metric should disclose.
If I could nominate one shadow metric for the list, it would be cohort payback time on a fixed-cohort basis — the companion that tells you whether that compounding base was cheap or expensive to build. Thanks for the calculator links too; pinning formulas next to definitions is the right instinct.