How to Calculate B2B SaaS Metrics: The Complete Formula Reference

Every B2B SaaS metric reduces to the same three ingredients — MRR, customer counts, and time — combined in different ratios to answer different questions. Retention metrics ask what happens to revenue you already have. Efficiency metrics ask what it costs to get more of it. Get the formulas right and the rest of your reporting stack (board decks, investor updates, pricing decisions) inherits that accuracy; get them wrong in the same spreadsheet you've been copying for two years, and every decision built on top is quietly miscalibrated.

This is a formula-first reference: what to calculate, the exact equation, and which calculator does it for you instantly. If you already know which metric you need, jump straight to its calculator — every one of them is free, runs in your browser, and scores your result against 2026 benchmarks.

Quick reference: every core SaaS metric formula

Metric	Formula	Calculator
MRR	Sum of all active subscriptions, normalized to monthly	MRR & ARR
ARR	MRR × 12	MRR & ARR
ARPU / ARPA	Total MRR ÷ active accounts	ARPU
Net revenue retention (NRR)	(Start MRR + Expansion − Contraction − Churn) ÷ Start MRR	NRR
Gross revenue retention (GRR)	(Start MRR − Contraction − Churn) ÷ Start MRR	GRR
Churn rate	Customers (or MRR) lost ÷ starting customers (or MRR)	Churn Rate
CAC	Sales + marketing spend ÷ new customers acquired	CAC
LTV	(ARPA × gross margin %) ÷ monthly churn rate	LTV
LTV:CAC ratio	LTV ÷ CAC	LTV:CAC
CAC payback period	CAC ÷ (ARPA × gross margin %)	CAC Payback
SaaS magic number	(Current qtr ARR − prior qtr ARR) × 4 ÷ prior qtr S&M spend	Magic Number
SaaS quick ratio	(New + Expansion MRR) ÷ (Churned + Contraction MRR)	Quick Ratio
Rule of 40	Revenue growth % + profit margin %	Rule of 40

Revenue metrics: MRR, ARR and ARPU

MRR is every active subscription normalized to its monthly value — annual contracts divided by 12, quarterly by 3 — and it's the base unit almost everything else is built on. ARR is simply MRR × 12, a run-rate projection, not a backward-looking GAAP revenue figure. ARPU (or ARPA at the account level) is MRR divided by active paying accounts, and it's the number that determines whether your price point can carry a self-serve motion or needs sales. Full walkthroughs and worked examples: MRR & ARR calculator, ARPU calculator.

Retention metrics: NRR, GRR and churn

Net revenue retention tracks one customer cohort's revenue over time — starting MRR plus expansion, minus contraction and churn, divided by starting MRR — and it's the single number investors check first, because it separates businesses that compound from businesses that just keep selling. Gross revenue retention is the same formula without expansion, capped at 100%, and it exposes what NRR can hide: heavy expansion from a few whale accounts masking a leaky base. Churn rate is the inverse view — customers or MRR lost divided by the starting base — and the one place operators most often get the math wrong is annualizing it: monthly churn compounds rather than multiplies, so 2.5% monthly is 26.2% annually, not 30%. See NRR, GRR, and churn rate for the full formulas and 2026 benchmark bands by stage.

Efficiency metrics: CAC, LTV, payback, magic number, quick ratio

CAC is fully loaded sales and marketing spend — salaries, ads, tools, commissions, not just media — divided by new customers won in the same period. LTV is margin-adjusted lifetime value: ARPA times gross margin, divided by monthly churn (since 1 ÷ churn approximates expected customer lifetime). Divide the two and you get the LTV:CAC ratio, where 3:1 or better is the standard efficiency bar. CAC payback period answers a related but distinct question — how many months of gross-margin-adjusted revenue it takes to earn back what you spent acquiring the customer — and should generally clear in under 12–18 months depending on segment. Two sales-efficiency metrics round this out: the SaaS magic number (quarter-over-quarter ARR growth, annualized, divided by the prior quarter's S&M spend) tells you whether it's time to scale sales spend, and the quick ratio (MRR gained ÷ MRR lost) tells you whether growth is efficient or just outrunning churn. Calculators: CAC, LTV, LTV:CAC, CAC payback, magic number, quick ratio.

The balance metric: Rule of 40

The Rule of 40 adds revenue growth rate to profit margin (usually free cash flow margin) and checks whether the sum clears 40% — the standard shorthand for whether you're allowed to burn cash because you're growing fast, or expected to run lean because you're not. Full breakdown, including the 1.33×-weighted variant investors sometimes use, at the Rule of 40 calculator.

Five mistakes that break these formulas

• Mixing time periods. Monthly ARPA paired with annual churn understates LTV by roughly 12×. Every input in a formula needs to share the same window.
• Revenue instead of profit. LTV, CAC payback, and break-even math all require gross-margin-adjusted revenue, not top-line revenue — skipping the margin adjustment overstates value by however much it costs you to serve the customer.
• Multiplying instead of compounding churn. Annualizing monthly churn by ×12 always overstates it. Use 1 − (1 − monthly churn)^12.
• Letting new customers into retention math. NRR and GRR are strictly same-cohort metrics — any new-logo revenue in the numerator inflates them into vanity numbers.
• Comparing against the wrong stage. A 90% GRR is enterprise-grade at Series B and unremarkable at seed. Benchmark against your stage and segment, not a single blended number — see the full band tables on the benchmarks page.

For a single scorecard that runs your core inputs through several of these formulas at once, try the founder metrics scorecard.