The Combined Ratio Explained: Underwriting Profit vs. Float Income

If you want to know whether an insurance company is actually good at insurance — as opposed to good at investing other people's money — the combined ratio is the single number to start with. It tells you, in one figure, whether the underwriting business is profitable before any investment income shows up.

The rule of thumb most investors learn: under 100% means the insurer makes money on underwriting; over 100% means it loses money on underwriting and is relying on float income to turn a profit. That's directionally right, but it hides a lot. Here's how to read it like an analyst.

How the Combined Ratio Is Calculated

The combined ratio is the sum of two pieces:

• Loss ratio = (claims paid + loss adjustment expenses) / premiums earned
• Expense ratio = (underwriting expenses + commissions) / premiums earned (or written, depending on the convention)

Add them together and you get the combined ratio. A combined ratio of 96% means that for every $1.00 of premium earned, the insurer paid out $0.96 in claims and expenses, keeping $0.04 as underwriting profit.

A few practical notes:

• Some insurers report on a net basis (after reinsurance); others on a gross basis. Net is the one that matters for shareholder returns.
• The expense ratio denominator varies — GEICO and direct writers tend to use earned premiums; commercial lines sometimes use written. Compare apples to apples within a peer group.
• Watch for prior-year reserve development. A reported combined ratio of 94% can include several points of favorable reserve releases from older accident years. The accident-year combined ratio strips that out and is closer to a real-time read on underwriting quality.

Underwriting Profit vs. Float Income: Two Very Different Businesses

Float is the money an insurer holds between collecting premiums and paying claims. For a fast-paying line like auto insurance, float turns over quickly. For long-tail lines like workers' comp or general liability, an insurer can hold premiums for a decade or more before paying them out.

That distinction matters because it changes what a "good" combined ratio looks like.

• Short-tail lines (auto, homeowners, property cat): Float is small relative to premiums. Investment income won't bail you out. You need a combined ratio meaningfully below 100% to earn a decent return on equity.
• Long-tail lines (workers' comp, casualty, reinsurance on liability): Float is enormous — sometimes 3x or 4x annual premiums. An insurer can run a 105% combined ratio for years and still produce a respectable ROE, because the investment yield on float covers the underwriting loss.

This is the structural reason Warren Buffett built Berkshire's insurance empire around long-tail reinsurance through General Re and National Indemnity. The cost of float (the underwriting loss) is small compared to what you can earn investing it. Progressive sits at the other end — short-tail auto, tight combined ratios in the low 90s in good years, much less float leverage per dollar of premium.

When a Sub-100% Combined Ratio Is Still a Bad Business

This is where most retail investors get the framework wrong. A 98% combined ratio is not automatically good news. Three things to check:

1. What's the interest-rate environment? A 98% combined ratio when 10-year Treasuries yield around current levels is a very different business than 98% when yields are much lower. Float compounds at the prevailing rate.
2. What's the reserve quality? If the insurer has been releasing reserves aggressively, the accident-year combined ratio may be 102% even though the calendar-year number reads 96%. That's a deteriorating business with a flattering headline.
3. What's the growth rate of written premium? Rapidly growing books are dangerous — claims show up later than premiums, so the loss ratio looks artificially low for a year or two. Insurers that chase growth in a soft market almost always pay for it 24-36 months later.

The related metric worth tracking is return on equity. An insurer running a 99% combined ratio with 3x float leverage and investment income at prevailing yields can earn strong ROE. Another running a 95% combined ratio with minimal float can struggle to hit reasonable return thresholds. The combined ratio is an input, not the answer.

A Quick Framework for Reading Any P&C Insurer

When you open a 10-K or quarterly release, walk through this checklist:

1. Calendar-year combined ratio — the headline number.
2. Accident-year combined ratio ex-cat — the underlying underwriting trend, stripped of catastrophes and reserve development.
3. Prior-year reserve development — is the company releasing or strengthening? Multi-year strengthening is a red flag.
4. Net investment income / float — what yield are they earning, and how does it compare to peers and to current new-money rates?
5. Premium growth vs. industry — fast growth in a soft pricing market usually ends badly.

Do that on two or three peers in the same lines and the differences in business quality become obvious quickly.

What to Watch Next

• Pull the latest 10-Q for an insurer you own and find the accident-year combined ratio ex-catastrophes. Compare it to the headline number — the gap tells you how much reserve development is flattering the result.
• Check the duration of the investment portfolio. As bonds roll over at current yields, float income should be changing for most insurers — quantify how much.
• For any insurer growing premiums faster than 10% annually, look at the 2-3 year lag: how did the accident-year loss ratio for those growth cohorts develop?
• Compare ROE across two peers with similar combined ratios but different float leverage. The structural difference in earning power will show up clearly.