How to Use This Calculator
- Choose °F or °C and pick a base temperature preset, or enter a custom one.
- Decide whether the 86/50 cap should apply — it’s on by default at base 50°F.
- Enter a single day’s high and low, or switch to Accumulation mode and add a row for each day.
- Read your growing degree days — in accumulation mode, watch the running cumulative total build as you add days.
What Are Growing Degree Days? (Also Called GDU)
Growing degree days — also called GDU, for Growing Degree Units — measure how much heat was available for a plant or insect to develop on a given day, above whatever minimum temperature its development actually requires. The two terms describe the exact same measurement; which one you hear tends to depend on the industry, not the math.
The formula is simple:
GDD = ((Tmax + Tmin) ÷ 2) − Tbase
Average the day’s high and low, then subtract the base temperature for whatever you’re tracking. If the result comes out negative — a day that never got warm enough to count — it’s recorded as zero, not a negative number, since development doesn’t run backward on a cold day. Add each day’s total to a running sum, and that accumulated GDD (or accumulated GDU) is the number you actually compare against a published threshold, like the pest emergence figures further down this page.
Start Date Matters
This is the part most calculators skip, and it’s the single easiest way to end up with a meaningless number: an accumulated GDD total means nothing without knowing when the counting began. Different published models use different start dates. Michigan State University’s GDDTracker starts counting on February 15. Iowa State’s growing degree day map suggests March 1 as a practical starting point, since GDD rarely accumulates meaningfully before then in most climates. Other models start on January 1.
These aren’t interchangeable. The same real weather produces a different cumulative total depending on which date you started counting from — a “500 GDD” reading measured from a February 15 start and a “500 GDD” reading measured from a January 1 start represent two different amounts of accumulated heat. Before comparing your total to any published threshold — the pest table below or anywhere else — check what start date that threshold assumes, and start your own count from the same date.
Choosing a Base Temperature
The base temperature is the point below which development effectively stops for whatever you’re tracking — degrees below it don’t subtract from the total, they just don’t add to it. 50°F is the standard base for most landscape and horticultural pests, and for most warm-season garden plants; it’s the right default if you’re not tracking something specific. 43°F shows up for some cool-season insects. 32°F is common in turf models, including the crabgrass pre-emergent threshold covered below, since it’s being used as a proxy for soil temperature rather than a biological development floor.
Use the preset buttons in the calculator above for these three, or enter a custom base if you’re working from a specific published threshold that names a different one.
Why the 86/50 Cap Exists
Above roughly 86°F, development for most tracked organisms doesn’t meaningfully speed up any further — and below 50°F, as covered above, it effectively stops. The 86/50 cap (sometimes called the modified GDD method) accounts for both ends: before averaging the day’s high and low, it treats any high above 86°F as 86°F, and raises any low below 50°F up to 50°F. A 92°F/45°F day and an 87°F/51°F day don’t actually represent that much more useful heat than a straight 86°F/50°F day would, and without the cap, extreme days quietly inflate the total.
Most free GDD calculators skip this step entirely and just average the raw numbers. The calculator above applies it by default whenever the base is set to 50°F — toggle it off if you’re working from a threshold that doesn’t use it, or on manually at another base if your source calls for it.
Pest Emergence Thresholds (Base 50°F)
These are common home-landscape pests with published GDD50 emergence windows — useful for timing scouting or treatment instead of guessing from the calendar. Every figure below assumes base 50°F except cabbage maggot, which is measured at base 43°F — note the different base carefully if you’re comparing your own accumulated total against it.
| Pest | Threshold |
|---|---|
| Codling moth | First moth emergence: 150 GDD50 |
| Cabbage maggot | 1st generation flies: 300 GDD43 — base 43°F, not 50 |
| Bronze birch borer | Larvae: 400–500 GDD50 |
| Black vine weevil | 1st-generation adults: 400–600 GDD50 |
| Bagworm | Egg hatch through early caterpillars: 600–900 GDD50 |
| Squash vine borer | Egg laying: 900–1,000 GDD50 |
| Apple maggot | First fly: 900 GDD50 · first egg laying: 1,100 GDD50 |
Source: University of Wisconsin Extension’s “Degree Days for Common Insect Pests”, except the bagworm figure, which comes from University of Massachusetts Amherst Extension and Rutgers Cooperative Extension’s Plant & Pest Advisory. These are reference midpoints, not guarantees — local populations and microclimates shift actual emergence by a week or more in either direction.
Crabgrass Pre-Emergent Timing
Crabgrass timing is where the “always state your base temperature” rule matters most, because two genuinely different published models are both in wide use, and quoting a number from one without its base is misleading.
Base 32°F model: apply pre-emergent between 250–500 GDD32 (Michigan State University Extension’s GDDTracker, citing Calhoun, MSU 2004). This is the model behind MSU’s Crabgrass Preemergence Timer, and it counts from February 15. It uses GDD32 as an indirect proxy for soil temperature rather than crabgrass’s own biological floor.
Base 50°F model: smooth crabgrass begins germinating around 150 GDD50, with large crabgrass needing roughly 150–200 more GDD50 on top of that (West Virginia University Extension). Kansas State University’s turf program cites a similar figure of approximately 200 GDD50 for germination onset.
Why the discrepancy? They’re not actually disagreeing — they’re measuring the same biological event on two different temperature scales, and a degree-day count on a 32°F base accumulates much faster than the same weather counted on a 50°F base, so the raw numbers aren’t interchangeable. A GDD threshold is only usable alongside its base temperature and start date — pick one model, use its base and start date consistently, and don’t mix a GDD32 reading against a GDD50 threshold or vice versa.
Timing a new lawn or overseeding job around this same spring window? See the Grass Seed Calculator for seeding rates, or the Overseeding Calculator if you’re thickening an existing lawn instead — both interact with pre-emergent timing, since most pre-emergent herbicides also block grass seed from germinating.
Worked Example
Three short examples, all at base 50°F, showing the formula, the zero-floor rule, and the 86/50 cap in action.
- A normal day: high 75°F, low 55°F. ((75 + 55) ÷ 2) − 50 = 15 GDD.
- A cold day: high 45°F, low 30°F. ((45 + 30) ÷ 2) − 50 = −12.5, which counts as 0 GDD — not a negative number.
- A hot day, with the cap: high 92°F, low 45°F. Capped, the high becomes 86°F and the low is raised to 50°F: ((86 + 50) ÷ 2) − 50 = 18 GDD. Uncapped, the same day would read ((92 + 45) ÷ 2) − 50 = 18.5 GDD — a small difference on one day, but one that compounds over a season of hot days.
Common Mistakes
- Comparing a total to a threshold without checking the base and start date. The single biggest source of wrong conclusions. A 400 GDD50 reading and a 400 GDD32 reading aren’t the same amount of heat, and neither is comparable across two different start dates. Always match both before comparing.
- Using base 50 for an organism with a different published threshold. Some pests and models genuinely use 43°F or 32°F — defaulting to 50 for everything produces a total that doesn’t line up with the actual research it’s meant to be checked against.
- Forgetting that negative days count as zero. A cold snap doesn’t subtract from an accumulated total — it just contributes nothing that day. Manually summing negative values instead of flooring them at zero will understate the real total.
- Expecting GDD to predict everything. It’s a heat model, not a complete one — moisture, day length, and soil fertility all also drive plant and insect development, and none of them show up in this formula. Use GDD to narrow a window, not to promise an exact date.
Growing Degree Days FAQ
What’s the difference between GDD and GDU?
Nothing, functionally — they’re the same formula producing the same numbers, just different dialects. GDU (Growing Degree Units) is the term the corn seed industry uses; GDD (Growing Degree Days) is the term used in horticulture and by university extension services. If you searched “what is GDU” expecting something different from GDD, the short answer is: it isn’t — use this calculator for either one.
What base temperature should I use?
It depends on what you’re tracking. 50°F is the standard base for most landscape and horticultural pests and for warm-season plants — start there if you’re not sure. Some cool-season insects use 43°F, and turf models (including the crabgrass pre-emergent threshold discussed below) commonly use 32°F. Whatever published threshold you’re comparing your total to, match its base — a GDD number without a stated base temperature isn’t usable.
When do you start counting growing degree days?
It depends on the model, and this trips people up more than the base temperature does. Michigan State University’s GDDTracker starts counting on February 15; Iowa State’s growing degree day map suggests March 1, since GDD rarely accumulates meaningfully before then in most climates; other models start January 1. There’s no universal default — use whatever start date the threshold you’re comparing against assumes.
How accurate are growing degree day predictions?
GDD is a heat model, and heat is one of several things driving development — it’s a useful predictor, not a perfect one. It doesn’t account for moisture, day length, or soil fertility, all of which also affect how fast a plant or insect actually develops. Use it to narrow a scouting or application window to a few days, not to pinpoint a single calendar date, and confirm with direct observation when the stakes are high enough to matter.
Sources & further reading
The GDD formula, the negative-days-count-as-zero rule, and general home-garden application draw on Iowa State University Extension’s “Using Growing Degree Days to Manage the Home Garden”. Pest emergence thresholds come from the University of Wisconsin Extension’s “Degree Days for Common Insect Pests” (bagworm figure from University of Massachusetts Amherst Extension). The base-32°F crabgrass model, its 250–500 GDD32 range, and the February 15 GDDTracker start date come directly from Michigan State University Extension’s GDDTracker. The base-50°F crabgrass germination figures come from West Virginia University Extension’s “Using growing degree days to predict germination”. For more on how we source and review the formulas behind every calculator on this site, see our methodology page and the full source list.