Anticipated Weather Days: How to Calculate Them for a Bid
Guess low and you eat the delay. Guess high and you lose the bid. The number you want is neither a gut feel nor a padded round figure — it is a defensible calculation you can hand to a GC and have them stop arguing.
Why "3 days a month" is costing you
Most subs carry a flat weather contingency — three days a month, ten days a job — regardless of whether they are grading clay in Virginia in March or pouring on sand in Arizona in June. That number is wrong in both directions, and it is indefensible in either.
It also quietly forfeits your strongest claim position. If you never stated what you anticipated, then when the GC asks "why is 11 days of rain abnormal?" you have no baseline to point at — and A201 §8.3 puts that burden squarely on you.
The three inputs that actually matter
1. Historical precipitation for the actual site
Ten years of daily records from the nearest official station, aggregated by month. Ten years is the convention: long enough to smooth out a freak season, short enough to reflect the current climate. Five years is noisy; thirty is arguably stale.
Use the site's ZIP, not the metro area. Rainfall varies enormously over short distances — a ridge line can be the difference between 38 and 46 inches a year.
2. What counts as a rain day for your scope
A "rain day" is scope-dependent, and this is where generic weather data fails you:
| Scope | Typical threshold | Also stopped by |
|---|---|---|
| Mass excavation / grading | ~0.25 in | Days of saturated ground afterward |
| Concrete flatwork | Any rain during pour/cure | Temperature, wind, evaporation rate |
| Structural steel | Rain is minor | Wind (crane picks) |
| Paving | Any rain; damp base | Ambient temp minimums |
If your contract defines a threshold, use theirs. If not, state yours in the exhibit so it is on the record from day one.
3. Soil — the input almost everyone omits
This is the difference between a number that survives scrutiny and one that does not. Rain lands on a day; the ground stays down for several. On poorly-drained clay a 1.5-inch event can mean three to four unworkable days. On sand, you may be back after lunch.
Count rainfall days only, and you understate an earthwork schedule by roughly half — then have to justify the gap later with nothing but "it was muddy." USDA/NRCS publishes soil survey data (drainage class, hydrologic group) for essentially every parcel in the country; it is free and it is authoritative. See how drying time actually works.
Putting it together
- Pull the baseline. 10-year daily precipitation for the site's ZIP, by month.
- Apply your threshold. Count days above it — that is your raw rain-day count.
- Add drying days. Adjust by soil drainage class, antecedent moisture, season, slope, and exposure. This is the step that turns 8 rain days into 13 lost days.
- Overlay your schedule. Only weather-sensitive, critical-path work counts.
- Write it down and submit it. An exhibit nobody saw at bid time is worth a fraction of one that was in the packet.
What to put in the exhibit
- Site location and the weather station used (with distance)
- Baseline period — e.g. "10-year average, 2016–2025"
- Month-by-month anticipated precipitation and lost days
- Soil classification and drying assumptions
- Your rain-day threshold definition
- The methodology, in enough detail that their scheduler can reproduce it
That last point is the whole trick. An exhibit that shows its work is hard to wave away. An exhibit that just asserts "14 days" is a negotiating position.
Generate the exhibit instead of building the spreadsheet
Enter a ZIP and dates. Ilystics pulls the 10-year NOAA baseline, looks up the USDA soil profile, applies the drying model, and returns a contract-ready Anticipated Weather Exhibit PDF with the methodology attached. Free, no account.
Generate a Free ExhibitIlystics provides weather and soil analysis, not legal advice. Thresholds and entitlement are contract-specific; consult your attorney or claims consultant.