Which financial ratios matter most for a small business?

Five cover the ground: gross margin (pricing vs. direct cost), net margin (overall profitability), current ratio (near-term solvency), accounts receivable days (collection speed), and debt service coverage (whether operating profit covers loan payments). Each answers a distinct question, comes from reports you already run, and moves for reasons an owner can act on.

How do I calculate debt service coverage ratio?

Divide operating cash income — commonly net income plus interest, depreciation, and amortization — by total annual debt payments, principal and interest. A ratio of 1.0 means earnings exactly cover payments; lenders typically want to see roughly 1.25 or better before extending new credit.

How often should I compute financial ratios?

Monthly, alongside the statement review — the value is in the trend, not any single reading. Compute the same five ratios the same way each month and chart them beside prior-year values; three consecutive moves in the wrong direction on any ratio is a finding even when each individual reading looks acceptable.

The five financial ratios worth a small operator's time

Textbooks list dozens of financial ratios; a small operator needs five. The test for inclusion is strict: the ratio must answer a question the owner can act on, be computable from the monthly statements without new data, and move for reasons the owner can trace. Gross margin, net margin, the current ratio, AR days, and debt service coverage pass. Most of the rest are analyst tools for comparing public companies.

The formula table

The five ratios, their sources, and rough small-business comfort zones (rules of thumb as of mid-2026, not laws):

Ratio	Formula	From	Comfort zone	The question it answers
Gross margin	(Revenue − COGS) ÷ Revenue	P&L	Stable vs. your own trailing average	Is pricing covering direct costs?
Net margin	Net income ÷ Revenue	P&L	Industry-shaped; stable or rising	Does the whole operation earn?
Current ratio	Current assets ÷ Current liabilities	Balance sheet	> 1.5	Can we pay the next few months' bills?
AR days	(Accounts receivable ÷ Revenue) × days in period	Both	Near your stated payment terms	How fast do customers actually pay?
Debt service coverage	(Net income + interest + depreciation) ÷ Annual debt payments	Both + loan schedule	> 1.25	Do earnings cover the loans?

What moves each one

Gross margin moves on pricing, discounting, supplier costs, and direct-labor efficiency — and on classification errors, which is why the COGS boundary in COGS vs. operating expenses must hold still for the trend to mean anything.
Net margin moves on everything gross margin does, plus overhead. When net falls while gross holds, the problem is below the gross line; the layered diagnosis is the subject of gross vs. net profit.
Current ratio moves on cash burn, inventory builds, and debt coming current — including the reclassification of long-term loan principal into current liabilities as maturities approach. Mechanics in working capital basics.
AR days moves on customer mix, invoicing speed, and collection discipline. If terms are net 30 and AR days reads 52, customers are taking three extra weeks of free financing from you. This ratio degrades earliest when a key customer weakens — it is the canary.
Debt service coverage moves on earnings and on the debt schedule itself. It is the ratio lenders compute first, generally wanting 1.25 or better; showing up with it precomputed is part of preparing lender-ready financials. The SBA publishes borrower guidance describing the same test.

Working the trend

Compute all five at each month-end close and keep a twelve-row running table. Reading rules: any ratio crossing its comfort boundary gets investigated this month; any ratio moving the same direction three months running gets investigated even inside the comfort zone. Consistency of computation matters more than formula elegance — a slightly crude AR-days formula applied identically every month beats a precise one applied intermittently.