Official skill · Finance

Unit economics check

v1.0.0 · Updated 4 November 2026

Checks CAC, LTV, LTV to CAC, payback, and gross margin from your numbers, shows each formula, and flags definitional mistakes that inflate results. Use it when before a fundraise, when a slide shows LTV:CAC or payback.

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When to use it

Before a fundraise, when a slide shows LTV:CAC or payback
When marketing and finance report different CAC

What you give it

Sales and marketing spend and new customers
ARPA and gross margin
Churn and how it was calculated

What you get

A table of their vs recomputed metrics with formulas, the definitional issues, and what matters most.

How it works

Recompute CAC CAC = sales and marketing spend ÷ new customers, same period. Show what spend was included. If only ad spend is included, call it "paid media CAC", not CAC. If paid and organic are mixed, also compute paid CAC = paid spend ÷ paid-acquired customers if the split is given.
Recompute gross margin Gross margin % = (revenue − cost of revenue) ÷ revenue. Check that hosting, third-party software in the product, payment fees, and customer support are in cost of revenue. If not, flag that margin is overstated.
Recompute LTV on gross margin Customer lifetime (months) = 1 ÷ monthly churn rate. LTV = ARPA per month × gross margin % × lifetime. If their LTV uses revenue without margin, flag it and show both. If churn is under 1% per month or based on fewer than 6 months of data, flag that lifetime is likely overstated.
Compute LTV:CAC LTV ÷ CAC. Reference point: a ratio above 3 is often cited for SaaS, and that guide assumes gross-margin LTV. State the ratio; don't call it good or bad without the user's own target.
Compute CAC payback Payback (months) = CAC ÷ (ARPA per month × gross margin %). Reference points: under 12 months is commonly cited as healthy for SaaS; 5 to 7 months is strong. Enterprise sales often run longer.
List definitional issues Check each: revenue LTV vs margin LTV; blended CAC hiding paid CAC; spend and customers from different periods; churn as a share of end-of-period customers instead of start; annual contracts counted as monthly; expansion revenue inside new-customer ARPA. Each issue gets the effect on the number.
Summarize Their figure vs your recomputed figure for each metric, and the 1 or 2 issues that matter most.

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