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
Frost-free growing days
188
Start seeds indoors by
2025-03-11
Safe outdoor transplant date
2025-04-22
Estimated harvest-ready date
2025-07-06
Fits before first fall frost?
Yes, fits before first frost
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter average last spring frost date.
- 2Enter average first fall frost date.
- 3Enter crop days to maturity.
- 4Enter weeks to start seeds before transplant.
- 5Enter safety buffer after last frost (days).
- 6Read your frost-free growing days on the right — it updates as you type.
- 7Hit Share to keep the scenario or send it to someone.
About this calculator
Your growing season length is the gap between the average last spring frost and the average first fall frost, and it determines which crops you can grow to maturity outdoors without protection. This calculator takes your last and first frost dates (available from NOAA climate normals or your local extension office at a specific probability, typically 50%), computes total frost-free days, and back-calculates safe indoor-seed-starting and transplant dates for a crop given its days-to-maturity and recommended weeks-before-transplant. Gardeners routinely lose a crop by planting warm-season vegetables like tomatoes or peppers two weeks too early, right into a lingering frost — this tool builds in your buffer explicitly instead of relying on the calendar.
Worked example
Using the values the calculator loads with:
Inputs
- Average last spring frost date: 2025-04-15
- Average first fall frost date: 2025-10-20
- Crop days to maturity: 75
- Weeks to start seeds before transplant: 6
- Safety buffer after last frost: 7 days
Results
- Frost-free growing days: 188
- Start seeds indoors by: 2025-03-11
- Safe outdoor transplant date: 2025-04-22
- Estimated harvest-ready date: 2025-07-06
- Fits before first fall frost?: Yes, fits before first frost
What each field means
Inputs
- Average last spring frost date
- The reference date for this calculation. Defaults to today unless you change it.
- Average first fall frost date
- The reference date for this calculation. Defaults to today unless you change it.
- Crop days to maturity
- The crop days to maturity used in the calculation. Starts at 75 so you have a working example on load. Accepted range: 20–200.
- Weeks to start seeds before transplant
- The weeks to start seeds before transplant used in the calculation. Starts at 6 so you have a working example on load. Accepted range: 0–12.
- Safety buffer after last frost (days)
- The safety buffer after last frost used in the calculation, measured in days. Starts at 7 days so you have a working example on load. Accepted range: 0–21 days.
Results
- Frost-free growing days
- Returned as a whole number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Start seeds indoors by
- Returned as a plain value. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Safe outdoor transplant date
- Returned as a plain value. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Estimated harvest-ready date
- Returned as a plain value. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Fits before first fall frost?
- Returned as a plain value. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Where do I find my local frost dates?
NOAA's National Centers for Environmental Information publishes 30-year climate normals by station and probability level (10%, 50%, 90% chance of frost). The 50% dates are the typical planting guide; use the 10% (later spring, earlier fall) dates if you want a more conservative, risk-averse window.
Why add a buffer after the last frost date instead of planting exactly on it?
The 'average' last frost date means frost still occurs after it in roughly half of years. A 7-14 day buffer for tender crops like tomatoes and peppers accounts for that variance; hardy crops like lettuce and peas can be planted before the average date with little risk.
What if my crop doesn't fit before first frost?
Shorten days-to-maturity by choosing an early variety, start seeds indoors earlier (increase weeks-before-transplant), use season-extension tools like row cover or a cold frame, or accept a fall harvest under frost protection instead of an open-field finish.
Does this account for microclimates in my own yard?
No — station data is a regional average. South-facing walls, urban heat islands, and low-lying frost pockets can shift your actual frost dates by 1-3 weeks from the official station reading, so treat this as a starting estimate to refine with your own observations.
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.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Frost Date Growing Window Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/frost-date-growing-window
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/frost-date-growing-window" target="_blank" rel="noopener">Frost Date Growing Window Calculator — RevenueLab</a> (2026).</p>
Source: [Frost Date Growing Window Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/frost-date-growing-window) (2026).