# Back-of-envelope math every fintech professional runs
A Series C fintech tells you: "We processed $2 billion in payments last quarter and grew 40% year-over-year." Sounds impressive. But is the business healthy? You have about two minutes before the next slide loads, and five calculations will tell you more than the entire pitch deck. This lesson gives you those five, cold.
Fintech disclosures are noisy by design. Founders lead with volume metrics (total payments processed, users onboarded) because they grow faster than profit metrics. Your job is to convert volume into unit economics in your head, fast. These five checks are what analysts at banks, VCs, and corp-dev teams actually run in a meeting, not in a spreadsheet later.
Take rate = Net revenue ÷ Total payment volume (TPV, the total dollar value processed).
Worked example: A payments company processes $2B in TPV and reports $30M in net revenue that quarter.
Take rate = $30M ÷ $2,000M = 1.5%.
Benchmark context (estimates, 2025-2026): card networks and payment facilitators like Stripe or Adyen typically run 1.5% to 3% take rates depending on mix (card-present vs. e-commerce, cross-border adds 1-2 points). Buy-now-pay-later (BNPL) players like Klarna or Affirm often show 4% to 7% because they're pricing in credit risk, not just processing. If a company claims payments-style margins but a BNPL-style take rate, ask why.
Before asking about company-wide profit, ask: does one unit of activity make money?
Contribution margin per transaction = (Revenue per transaction) − (Direct costs: interchange, processing, fraud losses, funding cost).
Example: A neobank charges no fees but earns interchange (the fee merchants pay, split between card networks and issuers, roughly 1.5-2% in the US under Regulation II caps for large banks, uncapped for small issuers under $10B in assets, the so-called "Durbin exemption"). If average interchange revenue per active user is $8/month and servicing cost (support, fraud, compliance) is $6/month, contribution is $2/month per user, before customer acquisition costcustomer acquisition costCustomer Acquisition Cost (CAC) is the total sales and marketing spend divided by the number of new customers gained in a period. It measures how efficiently you grow.View full definition →. That's the number that tells you if scale helps or hurts.
CACCACCustomer Acquisition Cost (CAC) is the total sales and marketing spend divided by the number of new customers gained in a period. It measures how efficiently you grow.View full definition → (Customer Acquisition CostCustomer Acquisition CostCustomer Acquisition Cost (CAC) is the total sales and marketing spend divided by the number of new customers gained in a period. It measures how efficiently you grow.View full definition →) = Sales & marketing spend ÷ new customers acquired in the period.
LTVLTVLifetime Value: the total revenue (or profit) a customer generates throughout their entire relationship with your business.View full definition → (Lifetime ValueLifetime ValueLifetime Value: the total revenue (or profit) a customer generates throughout their entire relationship with your business.View full definition →) = Average contribution margin per customer × expected customer lifespan (in months or years).
Simple version: LTVLTVLifetime Value: the total revenue (or profit) a customer generates throughout their entire relationship with your business.View full definition → = (monthly contribution margin) × (1 ÷ monthly churn ratechurn rateChurn rate is the percentage of customers or revenue lost over a period. It measures how fast a business loses its existing customer base.View full definition →).
Example: monthly contribution = $2, monthly churn = 4% (so average lifespan ≈ 25 months).
LTVLTVLifetime Value: the total revenue (or profit) a customer generates throughout their entire relationship with your business.View full definition → ≈ $2 × 25 = $50.
If CACCACCustomer Acquisition Cost (CAC) is the total sales and marketing spend divided by the number of new customers gained in a period. It measures how efficiently you grow.View full definition → = $40, LTVLTVLifetime Value: the total revenue (or profit) a customer generates throughout their entire relationship with your business.View full definition →:CACCACCustomer Acquisition Cost (CAC) is the total sales and marketing spend divided by the number of new customers gained in a period. It measures how efficiently you grow.View full definition → = 1.25x.
Benchmark (widely cited rule of thumb across SaaS and consumer fintech, treat as directional): 3x or higher is considered healthy; below 1x means the company is paying more to acquire customers than they'll ever return. Many consumer neobanks in 2024-2025 were reported (press estimates, not verified financials) to sit in the 1x-2x range during aggressive growth phases, which is why profitability timelines matter as much as growth rate.
Flagged explicitly: this is the one ratio from general banking that legitimately belongs in fintech, because any company holding loans on its own balance sheet (not just facilitating them) lives or dies by it.
NIM (Net Interest Margin) = (Interest earned on loans − interest paid on funding) ÷ average earning assets.
Example: A fintech lender earns $90M in interest income on a $1B loan book, and pays $30M to fund it (via warehouse credit lines or deposits).
NIM = ($90M − $30M) ÷ $1,000M = 6%.
Context (estimates): US fintech consumer lenders using warehouse facilities often target 5% to 9% NIM to cover credit losses and still profit; this is meaningfully higher than a traditional bank's NIM (roughly 3% for large US banks, per FDIC quarterly banking data) because fintech lenders serve thinner-file or subprime borrowers and price for higher default risk.
Burn multiple = Net cash burned ÷ Net new annual recurring revenueannual recurring revenueAnnual Recurring Revenue (ARR) is the normalized, predictable revenue a subscription business expects to earn from active contracts over a single year.View full definition → (ARRARRAnnual Recurring Revenue (ARR) is the normalized, predictable revenue a subscription business expects to earn from active contracts over a single year.View full definition →) added, over the same period.
Example: A fintech burns $40M in a year and adds $20M in net new ARRARRAnnual Recurring Revenue (ARR) is the normalized, predictable revenue a subscription business expects to earn from active contracts over a single year.View full definition →.
Burn multiple = $40M ÷ $20M = 2.0x.
Benchmark (popularized by investor David Sacks, widely cited across VC commentary): under 1x is excellent, 1x-1.5x is good, above 2x is a red flag in the current funding environment where capital efficiency, not just growth, decides valuations. This metric became the standard replacement for "growth at all costs" thinking after the 2022 rate-driven fintech valuation reset.
Knowledge check
1. A company reports strong total payment volume (TPV) growth but you want to assess how much of that flow actually becomes revenue. Which calculation should you run first?
2. A company presents itself as a straightforward payments processor, but its take rate is closer to what's typical for BNPL players than for card networks or facilitators. What should this discrepancy prompt you to ask?
3. Why do fintech founders typically lead pitch conversations with volume metrics (like TPV or users onboarded) rather than profit metrics?
4. Select ALL correct answers about why analysts convert a fintech company's headline volume numbers into per-unit or take-rate metrics during a live pitch.
Select all the correct answers.
5. Select ALL correct answers about what a contribution margin per transaction calculation is designed to reveal.
Select all the correct answers.
When a company hands you a claim, run this sequence:
1. Take rate first, sanity-checks whether the revenue model matches the sector (payments vs. lending vs. BNPL).
2. Unit economics second, is each transaction or account contribution-positive before overhead?
3. LTV:CAC third, is growth being bought at a sustainable price?
4. NIM, only if lending, is the credit engine itself profitable, separate from growth spend?
5. Burn multiple last, is the whole machine capital-efficient, or is growth masking a leaky bucket?
A useful gut check across all five: do the unit economics survive a rate environment shock? Fintech lenders funded via floating-rate warehouse lines saw NIM compress fast when benchmark rates rose in 2022-2023; ask what happens to NIM if funding costs move 200 basis points (2 percentage points).
Public disclosures worth knowing where to find: US-listed fintechs (Affirm, SoFi, PayPal) file 10-Ks and 10-Qs searchable via SEC EDGAR, which is free and where take rate, NIM, and provision expenses are stated line by line, not estimated. European players often disclose comparable metrics in IPO prospectuses or investor days (Adyen and Wise both publish detailed take-rate and volume breakdowns).
🎬 [VIDEO: "Unit Economics Explained" - youtube.com - search for CB Insights or a16z's public breakdowns of fintech unit economics and LTVLTVLifetime Value: the total revenue (or profit) a customer generates throughout their entire relationship with your business.View full definition →:CACCACCustomer Acquisition Cost (CAC) is the total sales and marketing spend divided by the number of new customers gained in a period. It measures how efficiently you grow.View full definition → frameworks for a visual walkthrough of these five calculations]