Seed Starting Date Calculator Guide
Use this seed starting date calculator to work backward from your last frost date and find when to sow indoors. Enter your estimated last spring frost and how many weeks before frost to start seeds for transplant-ready seedlings.
How to use the seed starting date calculator
Select your estimated last spring frost date from local extension maps or historical weather data. Enter how many weeks before that date to start seeds indoors — common values are 4–8 weeks depending on crop.
The result is your ideal indoor sowing date for transplanting after frost danger passes.
Seed starting date formula
Indoor sowing date = Last frost date − (Weeks before frost × 7 days). The weeks-before value comes from seed packet or extension charts for each crop.
- Sow date = last frost − (weeks × 7)
- Tomatoes and peppers: often 6–8 weeks before frost
- Cucumbers and squash: 3–4 weeks before frost
- Lettuce and brassicas: 4–6 weeks before frost
Worked example
Last frost is April 15 and your tomato seed packet says start 6 weeks before transplanting outdoors.
Sow date = April 15 − (6 × 7) = April 15 − 42 days = March 4. Transplant tomatoes after April 15 when nights stay above 50°F.
Finding your last frost date
USDA zone maps and county extension offices publish average last spring frost dates. Microclimates — south-facing walls, urban heat islands, elevation — can shift your date by 1–2 weeks.
When in doubt, wait an extra week before transplanting tender crops or use row cover for insurance.
Indoor seed starting tips
Use grow lights 14–16 hours daily and harden off seedlings outdoors for 7–10 days before transplanting. Pair with harvest date and succession planting calculators to map the full season.
- Use seed quantity calculator to buy enough seed for tray sowings.
- Bottom-water trays to prevent damping-off.
- Pot up seedlings that outgrow cells before transplant week.
- See frost protection planner for late cold snap contingencies.