The SaaS Metrics Every Founder Should Track (MRR, CAC, LTV, Runway)

Most early SaaS dashboards I see track the wrong things. Page views. Signups. Total revenue this quarter. Those numbers feel good and tell you almost nothing about whether the business is actually healthy.

The metrics that matter are the ones that answer four questions: Is recurring revenue growing? Can I afford to acquire customers? Are those customers worth more than they cost? How long until I run out of money? That's MRR, CAC, LTV, and runway. Get these four right and you can make almost every early-stage decision with confidence.

Here's how to calculate each one correctly — including the mistakes that quietly inflate them — with the actual formulas.

1. MRR — and the four movements inside it

MRR (Monthly Recurring Revenue) is the predictable revenue you can expect every month. The number itself is easy. What founders miss is that a flat MRR can hide a business that's bleeding.

Normalize everything to monthly first. An annual plan at $1,200 is $100 MRR, not $1,200 — never book the full annual amount as one month's MRR.

MRR = sum of each customer's monthly subscription value



Annual plan? MRR = annual price / 12

Then break the change in MRR into four movements — this is the part that reveals the truth:

Net New MRR = New + Expansion + Reactivation − Contraction − Churned



  New           = MRR from brand-new customers

  Expansion     = upgrades / seats added by existing customers

  Reactivation  = previously-churned customers who came back

  Contraction   = downgrades by existing customers

  Churned       = MRR lost from customers who cancelled

Two companies can both show "+$5k MRR this month." One did it with $5k of new sales and zero churn. The other added $15k of new sales while losing $10k to churn — a leaky bucket that will stall the moment sales slows. Only the breakdown tells them apart.

2. Churn — the metric that caps your growth

Churn is the single most important early signal. High churn means you're filling a bucket with a hole; no amount of marketing fixes it.

Customer churn rate = customers lost in period / customers at start of period



Revenue churn rate  = MRR lost (churn + contraction) in period / MRR at start

Track revenue churn alongside customer churn. If you lose small accounts but keep big ones, revenue churn looks great even when logo churn is ugly — and vice versa. When expansion revenue exceeds churned revenue, you have negative net revenue churn: the holy grail, where your existing customer base grows even if you add zero new customers.

Net Revenue Retention (NRR) = (starting MRR + expansion − contraction − churn) / starting MRR



NRR > 100% means the existing base is growing on its own.

3. CAC — what a customer actually costs

CAC (Customer Acquisition Cost) is the fully-loaded cost to win one customer. The common mistake is counting only ad spend.

CAC = (total sales + marketing spend in period) / new customers acquired in period

"Fully-loaded" means everything: ad spend, salaries of sales and marketing people, tooling, agency fees, content costs. If you only count the ad bill, your CAC looks half what it really is, and you'll overspend into a loss.

A useful companion is CAC payback period — how many months of gross margin it takes to earn back the cost of acquiring a customer:

CAC Payback (months) = CAC / (ARPA × gross margin %)



ARPA = average revenue per account per month

Under ~12 months is healthy for most B2B SaaS; under 6 is excellent. Long payback means your growth is financed by cash you don't get back for a year — dangerous when runway is short.

4. LTV and the ratio that decides if you have a business

LTV (Lifetime Value) is the total gross profit you expect from a customer before they churn.

Average customer lifetime (months) = 1 / monthly customer churn rate



LTV = ARPA × gross margin % × average customer lifetime

Always multiply by gross margin. Revenue isn't value — if it costs you 40 cents in hosting and support to deliver a dollar, only 60 cents is real. Skipping margin is the most common way founders fool themselves with a beautiful LTV.

Now the ratio that ties CAC and LTV together — the one a serious investor checks first:

LTV : CAC ratio = LTV / CAC



< 1   : you lose money on every customer. Stop and fix the model.

~ 3:1 : healthy. The standard target for sustainable SaaS.

> 5:1 : often means you're UNDER-investing in growth — spend more.

A 3:1 ratio means each customer returns three dollars of lifetime gross profit for every dollar spent acquiring them. Below 1:1, scaling just loses money faster.

5. Runway — how long the clock runs

Everything above is academic if you run out of cash. Runway is the number of months until you hit zero.

Net burn = monthly cash out − monthly cash in

Runway (months) = current cash balance / net burn

If you're burning $40k a month with $480k in the bank, that's 12 months of runway. Watch the trend, not just the snapshot: rising MRR shrinks net burn and extends runway every month — that's the whole game pre-profitability. Once cash in exceeds cash out, net burn goes negative and runway is effectively infinite. That's the finish line.

A worked example

Say you're at: 200 customers, $50 ARPA, 80% gross margin, 4% monthly customer churn, $600 CAC, $300k cash, $30k net burn.

MRR              = 200 × $50            = $10,000

Avg lifetime     = 1 / 0.04             = 25 months

LTV              = $50 × 0.80 × 25      = $1,000

LTV : CAC        = $1,000 / $600        = 1.67  → below target, fix churn or CAC

CAC payback      = $600 / ($50 × 0.80)  = 15 months → a bit long

Runway           = $300,000 / $30,000   = 10 months

The story those five numbers tell: revenue is real but churn is high (4%/month is steep), which drags LTV down so the unit economics are thin (1.67:1), and you have 10 months to fix it. That's a far more useful picture than "MRR is $10k and growing." The priority is obvious — attack churn first, because it improves LTV, the ratio, and payback all at once.

Put it in a model, not a memory

The mistake isn't not knowing these formulas — it's not having them wired into one live model where changing churn instantly updates LTV, the ratio, payback, and runway. When they're connected, you can ask "what if I cut churn to 2%?" and watch every downstream number move. That's when metrics become a decision tool instead of a report card.

If you want it pre-built

Wiring a clean, investor-ready model with all of these linked — MRR movements, cohort churn, LTV:CAC, payback, and a runway forecast that updates as you change assumptions — is a weekend of spreadsheet work if you start from scratch. I built one so you don't have to: the SaaS Metrics & Financial Model is a ready-to-use spreadsheet with every formula in this post connected, scenario toggles, and a START HERE guide that walks you through plugging in your own numbers in about ten minutes.

But you don't need it to start — the formulas above are the whole framework. Calculate your LTV:CAC and runway today; those two numbers alone will sharpen your next decision.

Which of these five do you actually track today, and which one have you been avoiding? Be honest in the comments — I suspect churn is the one most of us look away from.