Seed Starting Date Calculator
Calculate the date to start seeds indoors based on your last spring frost date and the weeks-before-transplant requirement.
How to use this tool
- Enter your average last spring frost date.
- Enter the number of weeks before transplanting to start seeds indoors (from the seed packet).
- The calculator returns the date you should sow seeds indoors.
Never miss your seed starting window — find the exact indoor sowing date for your garden.
Formula
Indoor sowing date = last frost date − (weeks before transplant × 7 days)
Computed using UTC calendar arithmetic to avoid daylight-saving-time anomalies.
How it works
This calculator subtracts a fixed number of days (weeks × 7) from the last expected spring frost date to find the date on which seeds should be started indoors, giving seedlings enough time to reach transplant size before it is safe to move them outside.
The weeks-before-transplant value is determined by the specific crop (e.g., tomatoes typically need 6–8 weeks, peppers 8–10 weeks); consult the seed packet or a regional planting guide for the correct value. Frost dates themselves are statistical averages — a 10–20% frost probability remains on the stated date in most climate references.
Worked example
- Inputs: last frost date 2025-04-15, 8 weeks before transplant.
- Days to subtract = 8 × 7 = 56 days.
- 2025-04-15 minus 56 days: April 15 − 56 = February 18, 2025.
Indoor sowing date = 2025-02-18.
Common mistakes to avoid
- Using the average last frost date for the whole region instead of the historical date specific to your ZIP code or microclimate, which can differ by two weeks.
- Interpreting 'weeks before last frost' as weeks before transplanting outdoors — some crops need hardening-off time after the calculated date before going outside.
- Starting seeds too early because the packet says a wide range (e.g. 6-10 weeks) and choosing the maximum, resulting in leggy transplants by the time the frost date arrives.
Key terms
- Last frost date
- The average calendar date after which freezing temperatures are unlikely in a given location; used as a reference point for spring planting schedules.
- Weeks before transplant
- The number of weeks a seedling needs to develop indoors before it is large enough to safely transplant into the garden.
- Hardening off
- The process of gradually exposing indoor-grown seedlings to outdoor conditions (temperature, wind, light) over 7–14 days before final transplanting.
- Frost probability
- The statistical likelihood of a frost occurring on or after the stated last frost date; many references use the date of 10% or 50% probability of frost.
- Transplant date
- The target date for moving seedlings from indoors to the garden, typically at or shortly after the last frost date for frost-sensitive crops.
Frequently asked questions
- When should I start seeds indoors?
- Most vegetables should be started 6–10 weeks before your last frost date. Tomatoes and peppers need 8–10 weeks; broccoli and cabbage need 4–6 weeks. Always check the seed packet.
- How do I find my last frost date?
- Enter your zip code on the USDA Plant Hardiness Zone Map website or ask your local cooperative extension service for the average last frost date in your area.