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Guide · ~7 min read
These two numbers decide whether an investor believes your go-to-market is a machine or a money pit. They're simple to compute and easy to overstate — so the whole game is computing them the way your investor does, and being honest about how sensitive they are.
CAC (customer acquisition cost) is your sales & marketing spend divided by the net-new logos it produced. CAC payback is how many months of gross profit per customer it takes to recover that cost:
CAC = $506,000 S&M ÷ 46 new logos = $11,000. Monthly gross profit = $2,400 ARPA × 74% margin = $1,776. Payback = 11,000 ÷ 1,776 = 6.2 months.
The mistake to avoid: dividing CAC by revenue instead of gross profit. Using $2,400 ARPA (not the $1,776 of gross profit) gives 4.6 months — a 26% understatement that vanishes the moment an investor asks "gross or net?" Always pay CAC back out of gross profit.
Rough read: under 12 months is generally healthy for seed/Series A B2B SaaS; under 6 is excellent; over 18–24 months suggests either CAC is too high or pricing is too low.
LTV (lifetime value) is the gross profit a customer throws off over their whole life. The simplest defensible version divides monthly gross profit by monthly revenue churn — because 1 ÷ churn is the average customer lifetime in months:
Monthly gross profit $1,776 ÷ 1.0% monthly churn = LTV $177,600. LTV:CAC = 177,600 ÷ 11,000 = 16.1×.
That 16.1× looks spectacular — which is exactly why investors don't trust it at face value. LTV:CAC is hypersensitive to churn, and churn is the input founders are most optimistic about:
| Monthly revenue churn | Implied lifetime | LTV | LTV:CAC |
|---|---|---|---|
| 0.5% | 200 mo | $355,200 | 32.3× |
| 1.0% | 100 mo | $177,600 | 16.1× |
| 2.0% | 50 mo | $88,800 | 8.1× |
| 4.0% | 25 mo | $44,400 | 4.0× |
Same CAC, same ARPA, same margin — the ratio swings from 32× to 4× purely on the churn input. It also silently assumes today's retention holds for the whole (often 8-year-plus) implied lifetime, which almost never survives contact with reality. A defensible 3×–5× with proven retention beats a fragile 16× built on an optimistic churn number, and experienced investors will tell you so.
Compute CAC payback and LTV:CAC from your inputs. Everything happens in your browser — nothing is sent anywhere.
CAC payback = CAC ÷ (ARPA × margin). LTV = (ARPA × margin) ÷ churn; LTV:CAC = LTV ÷ CAC. Same formulas the ModelKit engine uses. Illustrative method, not a benchmark or a valuation opinion.
Request the full model — $149The reason to build these off the same S&M line and ARR bridge as your P&L is that a partner will recompute them their way in the room. If your numbers were built on a separate tab with a friendlier churn input, the gap shows up immediately. ModelKit derives CAC payback and LTV:CAC from the same build as the statements, so there's nothing to reconcile.
ModelKit computes CAC, CAC payback, LTV:CAC, magic number and burn multiple from the same revenue build and P&L as your statements — so your GTM efficiency can't contradict your income statement.
Request the kit — $149 All the metrics →Educational material only — not investment, legal, financial, accounting, or tax advice, and not a valuation opinion. Figures are illustrative outputs of a deterministic method; benchmark ranges are common rules of thumb, not targets.