Planting Date Calculator: Last Frost to Sowing Date
When should I start seeds? Seed packets say things like "start indoors 6 weeks before the last frost" or "matures in 75 days". Enter your frost date or desired harvest date and this calculator gives the calendar date to plant. Frost dates are local averages, so use the ones for your area.
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Formula
- From a frost date:
planting date = frost date -/+ (weeks x 7 days), minus for "before" and plus for "after". - From a harvest date:
planting date = harvest date - days to maturity.
Calendar arithmetic is exact, including leap years. Frost dates and days to maturity are averages and vary by year and variety, so treat the result as a planning date rather than a guarantee.
Worked example
- Last frost 10 May 2026, start 6 weeks before: 6 x 7 = 42 days, so 29 March 2026 (a Sunday).
- Harvest 30 September 2026 with 75 days to maturity: 17 July 2026 (a Friday).
- First frost 5 October 2026, plant 9 weeks after: 63 days later is 7 December 2026.
- Last frost 15 April 2026, start 6 weeks before: 4 March 2026 (a Wednesday).
When to use it
Use it each season to turn the instructions on a seed packet into dates on a calendar. It works for starting seeds indoors, hardening off, transplanting, direct sowing and planning a late crop for autumn. Typical packet instructions are only a starting point: local conditions and your own experience matter more.
Common mistakes
- Using a frost date from a different climate zone. Ask your local extension service or weather service for averages.
- Forgetting whether days to maturity count from sowing or from transplanting.
- Treating the average last frost date as safe. There is always some chance of frost after it, so many gardeners add a week for tender plants.
- Counting calendar months instead of weeks. Six weeks is 42 days, not one and a half months.
How it differs from nearby calculators
The growing degree days calculator tracks accumulated heat rather than calendar days, which is a better fit for crops and insects that respond to warmth. The germination rate calculator tells you how many seeds to sow once you know when. The crop yield calculator is for after the harvest.
Frequently asked questions
How do I find my planting date from the last frost date?
Subtract (or add) the number of weeks on the seed packet from your average last frost date. With a last frost on 10 May and a start 6 weeks before, the date is 42 days earlier: 29 March.
How do I work out when to plant to harvest by a certain date?
Count back the days to maturity from the harvest date. For a crop with 75 days to maturity and a harvest wanted on 30 September, plant on 17 July. Days to maturity usually count from transplanting for tomatoes and peppers, and from sowing for direct-sown crops, so check which your packet means.
Where do I get my frost dates?
Your local extension service, weather service or garden centre publishes average last and first frost dates. They are averages, so there is always some chance of frost after the last-frost date. Many gardeners add a week of margin for tender plants.
Can I use this for fall crops?
Yes. Choose first fall frost and enter the weeks before it. A crop that needs 9 weeks before a first frost on 5 October should be planted by 3 August (63 days earlier). Choose weeks after to plan cold-tolerant plantings that follow the frost date.
When should I start tomato seeds indoors if my last frost is 15 April?
Tomatoes are commonly started about 6 weeks before the last frost, a typical packet instruction. Six weeks is 42 days, so 15 April 2026 minus 42 days is Wednesday 4 March 2026.
When should I plant to harvest on 15 October with a 60 day crop?
Count back 60 days from the harvest date. Choose the harvest mode with 60 days to maturity and a harvest date of 15 October 2026 to get Sunday 16 August 2026.
When can I plant tender crops after the last frost?
Choose last frost, set the direction to after and enter the weeks. Two weeks after a last frost on 10 May 2026 is 14 days later, which is Sunday 24 May 2026. Many gardeners add a margin like this for warmth-loving plants.