How Is Hail Size Measured?
Hail size is measured by the diameter of the hailstone, in inches, and is usually described by comparison to a familiar object — a quarter is 1 inch, a golf ball is 1.75 inches, a baseball is 2.75 inches. Radar estimates hail size remotely; ground observers confirm it. Hail at or above 0.75 inches is classified as severe.
The standard: diameter in inches
Hail is measured by its diameter across the widest point, reported in inches. The National Weather Service uses a set of familiar size comparisons so observers can report consistently:
- Pea — 0.25 inch
- Penny / dime — about 0.75 inch (severe threshold)
- Quarter — 1 inch
- Golf ball — 1.75 inches
- Hen egg — 2 inches
- Tennis ball — 2.5 inches
- Baseball — 2.75 inches
- Softball — 4 inches
Why 0.75 inches is the cutoff
The National Weather Service classifies a thunderstorm as severe when it produces hail of at least 0.75 inches (one inch since a 2010 standard change for warnings, though 0.75 inches remains the long-standing damage-relevant threshold in many datasets). Hail smaller than this rarely damages roofs. HailScore only counts hail at or above 0.75 inches for this reason.
Radar estimates vs. ground reports
There are two complementary ways hail size reaches the record:
- Radar estimation — products like MRMS MESH estimate the maximum likely hail size from radar returns above the freezing level. This covers everywhere, instantly, but is an estimate.
- Ground observation — trained spotters, the public (via CoCoRaHS), and NOAA Storm Events reports record what actually fell. This is ground truth, but only where someone was present to observe.
HailScore combines both: radar (NEXRAD and MRMS) for complete coverage, and ground-truth sources for verification.
Frequently asked questions
What size hail damages a roof?
Hail around 1 inch (quarter-size) can begin to damage asphalt shingles, and hail of 1.5 inches and larger frequently causes meaningful damage. Roof age and material matter — older and softer roofs are more vulnerable.
How is hail size estimated by radar?
Radar measures reflectivity above the atmospheric freezing level; the MRMS MESH product converts that into an estimate of the maximum likely hailstone diameter at each location.
Why do hail reports compare to balls and coins?
Because observers rarely have a ruler during a storm. Standardized object comparisons let the public and spotters report hail size consistently and quickly.
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