Business math for marketers

Mental math moves

The handful of tricks that make arithmetic feel easy. Everything else builds on these.

Round & adjust: 49 × 6? Do 50 × 6, then fix it.
Split it up: 23 × 7 = 20 × 7 plus 3 × 7.
Fives, 25s & halving: × 5 is just × 10, then halve.
Big numbers, lots of zeros: Impressions, budgets, TAMs. Don’t fear the zeros.

Percentages

The language of every dashboard. Learn these and you’ll catch mistakes other people miss.

X% of Y: Build any percent from 10%, 5% and 1%.
Percent change: From A to B: change ÷ where you started.
Points vs. percent: 2% → 3% is +1 point, but +50%.
Working backwards: $80 after 20% off? It wasn’t $96.
Stacked changes: Up 50% then down 50% isn’t back to even.

Funnels & ad metrics

Rates, ratios and how they chain together: the backbone of every GTM model.

CTR, CPC, CPM, CPA: Every ad metric is one division.
Funnel math: Conversion rates multiply.
Reverse the funnel: Need 50 customers? Work out the leads.
Where’s the lift?: Which fix moves the number most?

Growth & compounding

The idea that runs startups. Build the intuition that most people never get.

The rule of 72: Doubling time ≈ 72 ÷ growth rate.
Compounding growth: 10% a month for a year isn’t 120%.
Churn compounds too: 3% a month isn’t 36% a year.
CAGR: The one growth rate that explains a multi-year run.

Unit economics

CAC, LTV, payback, break-even. The numbers that decide if a business works.

CAC: What it really costs to win one customer.
LTV: Lifetime value = monthly value × how long they stay.
CAC payback: How many months until a customer pays you back?
LTV : CAC: Is each customer worth what you pay for them?
Break-even: How many sales until it pays for itself?

Data sense & estimation

Averages, samples, A/B tests and market sizing: know when a number is lying to you.

Blended averages: You can’t just average the averages.
How big a sample?: Margin of error ≈ 1 ÷ √n.
Is that A/B result real?: Noise ≈ √(conversions).
Market sizing (TAM): Companies × fit × price.