Calculate daily GDD and estimate crop maturity dates.
The GDD calculator measures plant development the way agronomists do — in accumulated heat units rather than calendar days. Crops mature when they have collected enough growing degree days, so a cool summer genuinely delays harvest and a warm one brings it forward. Enter today's high and low temperatures in °F, a base temperature (default 50°F, the standard base for corn and many warm-season vegetables), your cumulative GDD so far this season, and a target total; the tool returns today's GDD, the new accumulated total, and a projection of days remaining to the target.
Daily GDD = max(0, (high + low) ÷ 2 − base). The daily mean temperature is compared with the base temperature below which the crop makes no progress; days with a mean at or below base contribute zero — never negative — units. The new accumulated total adds today's GDD, and days-to-target = ⌈remaining GDD ÷ today's GDD⌉, a straight-line projection that assumes upcoming days resemble today. If today's GDD is zero the projection is undefined, and once the target is reached it reports zero days.
High 82°F, low 64°F, base 50°F → mean 73°F → 23 GDD today. With 800 GDD banked toward a 1,500 GDD variety, the remaining 700 units take ⌈700 ÷ 23⌉ = 31 more similar days.
High 75°F, low 55°F gives a 65°F mean and 15 GDD against the 50°F base. Strings of such days accumulate heat slowly — one reason spring-planted warm-season crops seem to stall in cool years.
High 60°F, low 40°F averages exactly 50°F — equal to the base, so GDD = 0. The crop neither gains nor loses units, and the days-to-target estimate becomes unavailable until warmth returns.
50°F is the conventional base for corn, beans, tomatoes, and most warm-season crops, and is this tool's default. Cool-season crops such as peas and lettuce are often tracked from about 40°F — the base field accepts any value.
Because development stops rather than reverses when it is cold. The formula clamps at zero with max(0, mean − base), matching the standard agronomic averaging method.
It assumes every future day matches today's GDD, so treat it as a same-weather projection rather than a forecast. Recompute as temperatures shift — in a warming spring the estimate will shorten week by week.
Seed catalogs and extension services publish heat-unit ratings — sweet corn varieties, for instance, are commonly rated in the 1,400–1,700 GDD range at base 50°F. Use the rating for your specific variety as the target input.