Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Expected weather delay days
40.0
Shortfall vs. scheduled contingency (+ = underprotected)
15.0
Unbudgeted cost exposure if shortfall
$13,500
Weather days already in schedule
25
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How to use this
- 1Enter avg adverse weather days per month (days).
- 2Enter project duration (months).
- 3Enter weather contingency days in schedule.
- 4Enter daily overhead rate (for cost estimate) ($/day).
- 5Read your expected weather delay days on the right — it updates as you type.
- 6Hit Share to keep the scenario or send it to someone.
About this calculator
Most contracts allow time extensions for weather days that exceed the historical average for the project location and season, which means estimators need a defensible baseline of expected 'normal' weather delay days built into the schedule from the start, not padded arbitrarily. This calculator takes the historical average number of adverse weather days per month for your region and season, multiplies by your project duration, and compares that expected delay against the number of weather contingency days you've actually built into the schedule, flagging a shortfall if your schedule float is thinner than historical norms suggest you'll need. It's a planning tool, not a claims tool — use it at bid time to size schedule contingency, and keep separate real-time weather logs during construction to support any actual time extension request against the contract's specific weather clause.
Worked example
Using the values the calculator loads with:
Inputs
- Avg adverse weather days per month: 4 days
- Project duration: 10 months
- Weather contingency days in schedule: 25
- Daily overhead rate (for cost estimate): 900 $/day
Results
- Expected weather delay days: 40
- Shortfall vs. scheduled contingency (+ = underprotected): 15
- Unbudgeted cost exposure if shortfall: $13,500.00
- Weather days already in schedule: 25
What each field means
Inputs
- Avg adverse weather days per month (days)
- The avg adverse weather days per month used in the calculation, measured in days. Starts at 4 days so you have a working example on load. Accepted range: 0–25 days.
- Project duration (months)
- The project duration used in the calculation, measured in months. Starts at 10 months so you have a working example on load.
- Weather contingency days in schedule
- The weather contingency days in schedule used in the calculation. Starts at 25 so you have a working example on load.
- Daily overhead rate (for cost estimate) ($/day)
- The daily overhead rate (for cost estimate) used in the calculation, measured in $/day. Starts at 900 $/day so you have a working example on load.
Results
- Expected weather delay days
- Returned as a decimal number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Shortfall vs. scheduled contingency (+ = underprotected)
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Unbudgeted cost exposure if shortfall
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Weather days already in schedule
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Where do I get a reliable average adverse weather days figure?
NOAA regional climate normals and local historical weather records for your specific project location and season are the standard source — a generic national average is nearly useless since a Gulf Coast project has vastly different rain-day patterns than a Mountain West project, and winter months everywhere run far higher adverse-day counts than summer months in the same region.
What counts as an 'adverse weather day' for contract purposes?
Most contracts define it as a day where weather conditions prevent normal work progress on the critical path — not simply any day with rain, but a day where actual measurable rain, snow, extreme temperature, or wind exceeds a contractually defined threshold and demonstrably impacts scheduled critical-path activities. Vague or undefined weather clauses are a frequent source of dispute over whether a given day actually qualifies.
How does this differ between above-grade and below-grade or exterior-heavy work phases?
Weather sensitivity varies dramatically by phase — earthwork, foundation, and building envelope phases are highly weather-exposed, while interior finish work once the building is enclosed is largely weather-independent. Apply your expected weather delay calculation primarily to the exterior-exposed phases of the schedule rather than spreading it evenly across the full project duration.
If my scheduled contingency exceeds the historical average, am I overpricing the job?
Not necessarily — historical averages are just that, averages, and a single bad season can exceed the average significantly (this is exactly what standard deviation and percentile data would show if available). A schedule with contingency slightly above the historical mean is prudent risk management, not padding, especially for exterior-heavy or winter-season work.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
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Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Weather Delay Days Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/weather-delay-days
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/weather-delay-days" target="_blank" rel="noopener">Weather Delay Days Calculator — RevenueLab</a> (2026).</p>
Source: [Weather Delay Days Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/weather-delay-days) (2026).