Hurricane category calculator

What "sustained wind" actually means

The number that sets a hurricane's category is the maximum one-minute average wind speed measured at 33 feet above the surface. That definition is specific to the United States, and it matters when comparing storms internationally: most other countries use a ten-minute average, which reads roughly 12% lower for the same storm. A system the National Hurricane Center calls a 100 mph hurricane would be reported nearer 88 mph under World Meteorological Organization convention.

Gusts are always higher than the sustained figure, typically by 20 to 25% over water and more over land, where friction and turbulence break the flow into peaks. A Category 3 hurricane with 115 mph sustained winds will produce gusts near 140 mph. Damage tends to track the gusts, but the category tracks the sustained speed.

Height matters more than people expect. Wind speed increases with altitude because there is less surface friction, which is why a high-rise apartment is a worse place to ride out a hurricane than a ground floor away from surge. In a strong storm, winds on the 30th floor can be a full category higher than at street level.

Why the scale dropped storm surge in 2010

The original 1971 scale included central pressure and expected storm surge ranges alongside wind. Both were removed, and the reason was that the correlation kept failing in ways that got people killed.

Hurricane Katrina in 2005 came ashore in Mississippi as a Category 3 and produced surge above 27 feet, well beyond anything the scale predicted for that category. Hurricane Charley in 2004 hit Florida as a Category 4 and produced a surge of about 7 feet, far less than the scale suggested. Katrina was enormous, so it had been pushing water toward the coast for days across a vast area; Charley was compact and fast-moving, giving the ocean little time to pile up.

Surge depends on the storm's size and forward speed, the angle of approach, the shape and slope of the continental shelf, local bays and river mouths that funnel water, and whether landfall coincides with high tide. A gently sloping shelf like the northern Gulf of Mexico allows far more water to stack up than a steep shelf like southeast Florida. None of that is captured by a wind speed, which is why the National Hurricane Center now issues separate Storm Surge Watches and Warnings with their own inundation maps.

Wind damage grows with the square of speed

The pressure wind exerts on a surface rises with the square of its velocity. In the formula US building codes use, pressure in pounds per square foot equals 0.00256 times the wind speed in mph squared. That squared relationship is the single most useful thing to understand about hurricane strength.

Work through it. A 74 mph Category 1 wind exerts about 14 pounds per square foot. A 130 mph Category 4 exerts about 43 — three times as much force, not 1.8 times as the speed ratio would suggest. At 157 mph, the Category 5 threshold, it is roughly 63 pounds per square foot, four and a half times the Category 1 figure.

Actual damage escalates faster still, because structures fail at thresholds rather than degrading smoothly. Once wind lifts a few roof shingles, water gets into the deck; once it peels back the sheathing, internal pressure rises and can push out walls from inside. Research from the Insurance Institute for Business and Home Safety has repeatedly shown that damage cost rises far more steeply than wind pressure alone — roughly a fourfold increase in loss for each category step. That is the real reason a Category 4 is not "somewhat worse" than a Category 2.

The category is the wrong thing to evacuate by

Emergency managers repeat one message every season: evacuate based on your zone, not on the category. Evacuation zones are drawn from surge modeling for your specific stretch of coast, using elevation and local geography that a single national scale cannot represent.

History backs this up in an uncomfortable way. Hurricane Sandy in 2012 was not even a hurricane at landfall by NHC classification, having transitioned to a post-tropical cyclone, and it killed more than a hundred people and caused around 70 billion dollars in damage — because it was enormous and hit at high tide during a full moon. Tropical Storm Allison in 2001 never reached hurricane strength and caused catastrophic flooding in Houston.

Freshwater flooding from rainfall, not wind or surge, has caused the majority of US tropical cyclone deaths in recent decades. Rainfall totals depend mostly on how slowly the storm moves and how much moisture it carries, which the category says nothing about at all.

The formula

Saffir-Simpson Hurricane Wind Scale (sustained wind, mph)

  Tropical Depression    below 39
  Tropical Storm            39 – 73
  Category 1                74 – 95
  Category 2                96 – 110
  Category 3               111 – 129   ← "major" begins here
  Category 4               130 – 156
  Category 5               157 and above

Wind pressure, from ASCE 7 as used in US building codes:
  q = 0.00256 · V²      q in lb/ft², V in mph

The category boundaries are defined in knots and converted, which is why the mph figures look arbitrary: Category 1 begins at 64 knots, Category 3 at 96 knots, and Category 5 at 137 knots. There is no Category 6 — the scale is open-ended at the top because damage is already described as catastrophic, and adding a category would not change any protective action.

Practical tips

• Know your evacuation zone before the season starts. Every coastal county publishes a map, and looking it up during a warning is too late.
• Prepare for a week without power, water or open stores after any major hurricane. One gallon of water per person per day is the standard figure.
• Board up or install shutters before the wind arrives. Tropical storm force winds reach the coast many hours ahead of the eye, and outdoor work becomes impossible well before landfall.
• Fill your car's tank and get cash early. Fuel pumps and card readers both need electricity.
• Photograph your home and belongings before the storm. It makes insurance claims dramatically easier afterwards.

Frequently asked questions