Sales by Customer: Finding Your 80/20

The Pareto principle — 80% of effects come from 20% of causes — shows up almost everywhere in business. Sales by customer is one of its most consistent manifestations. The exact ratio varies, but the concentration of revenue in a small number of customers is real and worth knowing. Once you can see it clearly, several important decisions become easier.

How to do the analysis

The mechanics are straightforward:

• List every customer with their total sales for the period (last 12 months is the standard window)
• Sort descending by sales value
• Compute the cumulative share of total revenue as you go down
• The customers needed to reach 80% of revenue are your "A" customers; the next ~10–15% are "B"; the long tail is "C"

The exact percentage thresholds matter less than the shape of the curve. A steep curve where 5% of customers drive 60% of revenue is a very different business from a flat one where 50% drive 60%.

What you usually find

In B2B businesses, the concentration is typically extreme: 10–20% of customers driving 70–80% of revenue. Sometimes a single top customer is 20% or more on its own.

In B2C, the concentration is less extreme but still real: 20–30% of customers driving 60–70% of revenue (when repeat buyers are counted as the same customer).

The exact numbers per business need to be calculated. The default assumption that "all customers are roughly equal" is almost always wrong.

Why this matters: risk

Customer concentration is a risk factor. If your top 10% accounts for 70% of revenue and any one of those customers leaves — for any reason — the impact on the business is severe and immediate.

Some checks worth doing on the top 10:

• Is the relationship secure (long-term contract vs at-will)?
• Is there a single point of contact whose departure could end the relationship?
• Are you their primary supplier or one of several?
• Are they themselves financially healthy?
• What is their growth trajectory — growing with you or shrinking?

Concentration is not bad in itself — many great businesses have it. But it is a risk that should be visible, monitored, and offset where possible.

Why this matters: profitability

Customer concentration is also an opportunity, but with a twist. Your top customers are not always your most profitable customers. Common patterns:

• Top customers often get the best pricing (volume discounts) — high revenue but lower margin per rupee
• They often demand more service — dedicated support, custom terms, faster delivery
• They often have more leverage — payment terms longer than your average, more disputes
• Their orders may be larger but lumpier — cash flow swings

The right number to look at is not just sales by customer but contribution margin by customer — revenue minus direct cost minus customer-specific costs (extended payment terms, dedicated service, custom packaging, etc.).

Sometimes the most profitable customers are in the middle of the distribution — large enough to be material, not so large that they extract every concession.

Why this matters: focus

Once you can see customer concentration, your time allocation should reflect it. Most sales teams spread effort roughly evenly across customers — which means the A customers (where the revenue is) get the same attention as C customers (where it isn't).

A deliberate reallocation:

• A customers — named account management; quarterly business reviews; senior relationship touchpoints; deep understanding of their needs
• B customers — regular touchpoints; cross-sell / up-sell focus; potential to graduate to A
• C customers — efficient self-service; automation; low-cost service model; do not let them consume the time A and B deserve

The freed-up time on A and B usually produces measurable revenue growth.

The receivables angle

There is one more dimension worth checking: customer concentration in receivables. If your top 10% by sales is also your top 10% by overdue receivables, that's a different (and more urgent) concentration to manage.

A customer that buys a lot but pays slowly is funding their growth on your working capital — see our receivables ageing post for what to do about it.

The growth pattern: where new revenue is coming from

A useful follow-on analysis: split sales growth by source:

• Same A customers, larger orders — deepening the relationship
• New A customers won this period — landing big new accounts
• B customers graduating to A — the development pipeline
• C customers in aggregate — the long-tail behaviour
• Lost customers (revenue from customers active last year but not this year)

The mix tells you whether the business is growing through deeper penetration, new accounts, or churn replacement. Each pattern calls for a different sales strategy.

How often to re-run

Quarterly is the right cadence for most businesses. Run the analysis, compare to last quarter:

• Are A customers staying A?
• Are any C customers showing signs of being A in the making?
• Has the concentration ratio increased or decreased?
• Who has dropped off entirely?

Year-over-year, the same comparison at a slower cadence. The trend matters more than the snapshot.

How Booksmor helps

Booksmor produces customer ABC analysis from your sales data — by revenue, by gross margin, and by contribution margin (when customer-specific costs are tracked). Concentration trends, new-A landings, and customer churn are surfaced quarterly. Start a 30-day free trial and put real customer analytics in front of your team.