Appendix A — SEO Metrics, Formulas, and CTR Models

The math in SEO is simple arithmetic, but it is the arithmetic that turns "we rank better" into "we made more money" — the translation every strategist has to be able to do (Chapters 29, 39). This appendix collects the handful of relationships worth memorizing. Every number in the worked examples is illustrative, chosen to show a method; when you run these on a real site you will use its real data.

1. The master identity: from ranking to revenue

Organic search value builds up in one chain. Learn it once and you can answer almost any "what's it worth?" question.

$$\text{Revenue} = \text{Impressions} \times \text{CTR} \times \text{Conversion rate} \times \text{Value per conversion}$$

Impressions — how many times your listing appears (driven by how many people search × whether you rank at all).
CTR (click-through rate) — the share of impressions that become clicks (driven mostly by position and by title/description quality — Chapter 9).
Conversion rate — the share of visitors who take the valuable action (call, form, booking, purchase).
Value per conversion — average revenue (or lead value) per conversion.

Worked example (all illustrative): a query with 2,000 monthly searches, your page at a position earning a 10% CTR, a 5% conversion rate, and \$400 value per booked job:

2,000 × 0.10 × 0.05 × $400 = $4,000/month of organic value from a single query. That is why one page can matter.

2. The CTR-by-position curve (use the shape, never a fake precise number)

Click-through rate falls steeply as position drops. This is the single most important pattern in SEO economics, and it is why the difference between #1 and #11 is enormous even though both are "ranking."

CTR BY ORGANIC POSITION (illustrative shape — NOT measured values)
  #1   ██████████████████████████████  (by far the most)
  #2   ████████████████
  #3   ███████████
  #4   ████████
  #5   ██████
  #6   ████
  #7   ███
  #8   ██
  #9   ██
  #10  █
  ─────────────────────────────────────
  page 2 (#11+)  ▏ (a rounding error for most queries)

Honesty warning. You will see confident figures like "#1 gets 31.7% of clicks." Do not quote them as fact (Chapter 1's Evidence Check). Real CTR varies enormously by query type, by how many ads and SERP features crowd the page, by brand vs. non-brand, and by year. Use the curve's shape — steep decline, a cliff after position 3–4, near-zero on page 2 — and pull your own CTR-by-position data from Search Console (Chapter 27) when you need real numbers.

3. Traffic forecasting (what a ranking gain is worth)

To estimate the value of improving a page's position:

$$\Delta\text{Clicks} \approx \text{Search volume} \times (\text{CTR}_{\text{new position}} - \text{CTR}_{\text{old position}})$$

Worked example (illustrative CTRs): moving a 2,000-searches/month term from position 8 (≈2% CTR) to position 3 (≈10% CTR):

2,000 × (0.10 − 0.02) = 160 extra clicks/month — before you write a word of new content, so you can decide whether the work is worth it. Multiply by conversion rate and value to get dollars (Section 1).

Cautions: search volumes from tools are estimates (often rounded/averaged); CTR assumptions are just that; and you cannot control whether you actually reach position 3. Forecast in ranges, label them clearly, and never present a forecast as a promise (Chapter 39).

4. SEO ROI

$$\text{ROI} = \frac{(\text{Organic revenue} - \text{SEO cost})}{\text{SEO cost}}$$

The distinctive feature of SEO ROI is timing and compounding: costs are front-loaded and returns arrive months later, then persist and grow as authority accrues — unlike paid search, whose traffic stops the moment spending stops (Chapter 39). A one-month snapshot almost always understates SEO; evaluate it over 6–12+ months.

5. Reading the metrics you'll be handed

Metric	What it is	How to read it honestly
Search volume	Estimated monthly searches for a term	A vendor estimate, often rounded and averaged over 12 months; directional, not exact (Chapter 7).
Keyword difficulty	A 0–100 score of how hard a term is to rank for	A third-party tool's estimate (usually from competitors' link metrics) — not a Google number; compare within one tool, don't cross-compare (Chapter 7).
CPC	Average cost-per-click in ads	A useful proxy for commercial value — high CPC means the term makes money (Chapter 7).
Domain Authority / Domain Rating	A 0–100 third-party "authority" score	A third-party metric, not used by Google (Chapter 22). Useful for comparing sites in one tool; meaningless as an absolute target ("raise our DA" is a vanity goal — Chapter 29).
Impressions / Clicks / CTR / Average position	From Search Console	The closest thing to ground truth you have — it's your data from Google (Chapter 27). High impressions + low CTR = a title/description opportunity.
Engagement rate / conversions	From GA4	What ties traffic to value; the metrics that matter to stakeholders (Chapters 28–29).

6. The KPIs that matter vs. the vanity metrics (quick reference)

Report these (they connect to the business): organic sessions, organic conversions and their value, rankings for priority terms, indexed pages, Core Web Vitals status, growth in referring domains.

Distrust these as goals (context-free): total number of keywords ranked, third-party Domain Authority, raw backlink counts, "impressions" without clicks, and any single vanity number a tool puts in a big font.

Keep this appendix beside Chapters 29 (reporting) and 39 (the business case); together they turn measurement into the argument that funds the work.