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Customer Concentration Calculator: Top-client Dependency Risk

Find out how dependent you are on a few clients. Enter revenue for each customer (group the rest as “All others”) to see top 1, top 5 and top 10 shares, the HHI concentration index and a risk verdict.

QUICK ANSWERCustomer concentration is the share of revenue from your largest customers. A single customer above ~20–30% of revenue, or top 10 above ~50%, is usually treated as high risk in diligence. The HHI index (sum of squared shares) above 2,500 indicates high concentration.

Share = Customer revenue ÷ Total revenue
HHI = Σ (share %)²

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What the Customer Concentration Calculator does

✅ Top 1, 5 and 10 share
✅ HHI index
✅ Risk verdict
✅ Revenue share bars
✅ Handles an “All others” row

How to use the Customer Concentration Calculator

STEP 1
Enter your numbers
Type the figures you know. Every field starts with a realistic example so you can see how it works.
STEP 2
Read the result instantly
Results, tables and the plain-English explanation update as you type. No submit button, no login.
STEP 3
Test scenarios
Change one assumption at a time (valuation, growth, dilution, exit) to see what really moves the outcome.
STEP 4
Share or save
Copy the summary, send it on WhatsApp or print to PDF for your co-founders, board or investment committee.

Living with a big customer

Early B2B startups often depend on one or two anchor customers. Investors accept it if contracts are long, renewals are proven and the pipeline shows diversification coming.

Group customers under the same parent (for example, several subsidiaries of one group) when measuring concentration; diligence teams will.

Frequently asked questions

What is high customer concentration?

Typically one customer above 20–30% of revenue or the top 10 above about 50%.

What is the HHI index?

The Herfindahl-Hirschman Index: the sum of the squares of each customer’s percentage share. It ranges up to 10,000 (one customer).

How do investors treat concentration?

They may lower the valuation multiple, ask to speak with the top customers, review contracts and termination clauses, or structure part of the round on milestones.

Is this tool free? Do I need to sign up?

Yes. It is completely free, needs no login and has no usage limit. FastFee builds payment collection software; these startup and investor tools are free resources for founders, angels and fund managers.

Is my data private?

Yes. Everything you type stays in your own browser on this device (local storage). Nothing is uploaded to FastFee. Export or print a copy if you need to keep it.

Use it together with

Revenue Quality Score →Score revenue on recurring share, contract length, concentration, margin, retention and DSO.Startup Red Flag Detector & Claim Verifier →Flags unusual growth, margins, runway, concentration and cap table issues, with how to verify each claim.

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