What is a common-size income statement?

A P&L in which every line is divided by total revenue and shown as a percentage — revenue is 100%, and each cost appears as the share of every revenue dollar it consumes. It makes periods of different sizes directly comparable and exposes cost drift that dollar figures hide.

Why use percentages instead of dollar amounts to compare years?

Because growth camouflages drift. Rent that rose from $48,000 to $54,000 while revenue grew 40% actually fell as a share of revenue; payroll that 'only' grew with revenue may have crept from 30% to 34% of it. Percentages hold the yardstick constant when the business changes size.

How much percentage drift is worth investigating?

A reasonable small-business rule: investigate any line that moves more than one percentage point of revenue year over year, or any trend moving the same direction three periods running. One point of revenue is real money — at $600,000 of revenue, a one-point drift is $6,000 a year.

Common-size statements: read every line as a percent of revenue

A common-size income statement divides every line by total revenue: revenue becomes 100%, and each cost becomes the share of every revenue dollar it consumes. The format solves the fundamental problem of comparing periods when the business changed size — a $6,000 rise in payroll means nothing by itself, but payroll moving from 31% to 34% of revenue means the business is becoming structurally more expensive to run, whatever the dollar columns say.

The format, in one table

An illustrative studio, two years, dollars and common-size side by side:

Line	2024	%	2025	%	Drift
Revenue	480,000	100.0	620,000	100.0	—
COGS	206,000	42.9	285,000	46.0	+3.1
Gross profit	274,000	57.1	335,000	54.0	−3.1
Payroll (admin)	96,000	20.0	124,000	20.0	—
Rent & utilities	48,000	10.0	54,000	8.7	−1.3
Marketing	19,000	4.0	37,000	6.0	+2.0
Other operating	43,000	9.0	56,000	9.0	—
Net income	68,000	14.2	64,000	10.3	−3.9

The dollar columns tell a growth story — revenue up 29%. The percentage columns tell the real one: gross margin surrendered three points and marketing took two more, so a bigger business earned less than the smaller one, in dollars and dramatically so as a share of revenue. Every question worth asking this year is in the drift column.

Reading rules

Anchor on gross margin first. It is the largest and most diagnostic percentage; the boundary that defines it is covered in COGS vs. operating expenses, and its meaning in gross vs. net profit.
Investigate one-point movers. Below gross margin, most lines should hold within a point of revenue year over year. A one-point rule scales naturally with the business — no threshold updating needed.
Respect direction over size. A half-point drift repeated three years is a trend; a two-point spike in one year is often an event. Trends are the dangerous ones because no single year forces a decision.
Fixed costs should shrink as percentages when revenue grows. Rent falling from 10.0% to 8.7% above is what operating leverage looks like. Fixed lines that hold their percentage during growth are quietly becoming variable costs — worth asking why.

Where to get it

Most small-business ledgers produce the format natively — the P&L report typically offers a "% of income" column — so this is a setting, not a spreadsheet project. Run it annually against the prior year at minimum, and quarterly if margins are tight. It pairs naturally with the dollar-based variance review in budget vs. actual reports: budget-versus-actual catches events, common-size catches drift.

One caution on comparability: the percentages are only meaningful if classification held still. A year in which direct labor moved between COGS and overhead, or the books changed accounting method (the methods themselves are covered in Publication 538), will show phantom drift that is really reclassification. Note such changes on the statement itself, the way Publication 583 urges consistent records generally — future you, reading the trend, will not remember.