How accurate is the snow day calculator?
Short answer: its high numbers hold up and it misses a fair share of closures. We set it against what 17 US school districts really did over two winters, 2,024 school days in all. When it said 80% or more, schools closed 86% of the time. It also gave under 20% to about three closures in ten.
What the percentage means
A result of 70% means the forecast looks like the mornings on which schools in a district like yours usually close. The rules are written down by people and have had one check against real closings, described below. Two things sit behind every number:
- The forecast. If the storm arrives three hours late or drops half the snow, the estimate was built on the wrong night.
- The rules. Thresholds such as "six inches overnight is a coin flip where snow is common" are ours. They are listed on the How it works page so you can disagree with them.
What the check found
The districts run from Atlanta and Dallas to Minneapolis and Portland, Maine. For each school day from December to mid-March of 2024-25 and 2025-26 we took the forecast as it stood a day ahead, ran the calculator, and compared the answer with what the district announced: 2,024 school days, of which 119 were closed or moved online for weather.
| The calculator said | School days | Closed or online |
|---|---|---|
| Under 5% | 1,761 | 1% |
| 5% to 20% | 131 | 18% |
| 20% to 50% | 50 | 32% |
| 50% to 80% | 23 | 65% |
| 80% or more | 59 | 86% |
- High numbers can be trusted. Of the 1,871 ordinary school days, 23 were given 30% or more.
- Low numbers are too low. A result of 10% went with a closure about one time in six.
- The days after a storm were the weak spot. Roads stay icy and power stays out after the forecast has cleared. The calculator now carries a share of a warning into the next three days, which caught some of these closures and not all.
- Weather service warnings matter most. With a winter warning in force that morning, schools closed on 51 days of 57. The first version gave warnings too little weight; that is what this check changed.
Read these figures with care. The weight of warnings was set on the winter of 2025-26 and then scored on 2024-25, a winter it had not seen, and it held up there. The share carried into later days was then set on both winters, so the table is partly scored on the days it was tuned on, which flatters it. The record for 2024-25 is the thinner of the two: 67 days we could not confirm either way were left out. The archived forecast we could get is not the National Weather Service forecast the calculator reads live, and it records ice less well. Canadian closings have not been checked at all.
When to trust it more, and less
| When you check | Confidence shown | Why |
|---|---|---|
| The evening before, or that morning | High | Timing and totals are as firm as a forecast gets |
| A day to a day and a half ahead | Medium | The storm's track can still shift |
| Two or more days ahead | Low | Start times and amounts often change by then |
Confidence here is about the forecast's lead time only. It is not a measure of how often the calculator has been right.
What it cannot see
- Snow and ice already on the ground, when a storm ended before the forecast was issued.
- How fast road crews work in your town, and whether buses will start.
- Your district's habits: some superintendents close early and often, some almost never.
- Remote learning days, used snow days, power outages and building problems.
What would make it better
More winters and more districts, so that the figures above lean less on a few big storms. A better way to tell when roads and power are back after a storm. And the same check for Canada. Each time the rules change, this page gets the new figures, including the ones that look bad.
Common questions
Is the snow day calculator real?
Yes, in the sense that it does a real calculation on the real forecast for your location, by rules that are published in full. No, in the sense some sites imply: it does not know what your superintendent will decide, and neither does any other calculator.
How accurate is the snow day calculator?
We checked it against two winters in 17 US school districts: 2,024 school days, 119 of them closed or moved online. When it said 80% or more, schools closed 86% of the time. It still gave under 20% to about three closures in ten. Two winters is a small sample, so read it as a check, not a guarantee.
When is the estimate most reliable?
The evening before and early on the morning itself. Storm timing and snowfall totals in a forecast are firmest in the last day before the storm. The calculator shows a confidence line under the sign that drops from high to low as the school morning gets further away.
Why did the calculator say 80% and school stayed open?
An 80% chance still leaves one morning in five open. Beyond that: the forecast may have missed, the roads may have been treated faster than usual, or your district may simply close less readily than the setting assumes. Change the district setting if it keeps overshooting.
What is the most accurate snow day calculator?
Nobody has published a comparison between calculators, so there is no honest way to name one. Look for a calculator that shows its reasons, names its forecast source and tells you when the forecast was issued, and treat all of them as estimates.