Tool
Seed starting date calculator
Count back from your last expected frost to find the right week to sow indoors.
Your result
Sow indoors around 20 March 2027
- Last frost date
- Weeks before
What this calculator does
Many crops are sown indoors weeks before they are planted outside, and the timing is set by your local last frost date. This tool subtracts the number of weeks your crop needs from that date, giving an approximate sowing day. It is a planning aid, not a guarantee — weather and indoor conditions still decide how fast seedlings grow.
When to use it
- Planning your indoor sowing for the year.
- Working out succession sowings so crops mature at different times.
- Checking whether a seed packet's "sow in March" fits your climate.
How to use it
- Find your last frost date. Your national weather service or a local gardening calendar gives a typical date for your area.
- Check the seed packet for how many weeks before that the crop should be started.
- Enter both above to get a sowing date.
- Adjust to your setup. A warm windowsill or heat mat speeds growth; a cold greenhouse slows it.
Worked example
Your last frost is around 15 May, and the tomato packet says sow 8 weeks before. Eight weeks back from 15 May gives around 20 March — your indoor sowing week. If you have no heat and a cool windowsill, sow a week or two later so the seedlings are not ready before the weather is.
Typical weeks before last frost
| Crop | Start indoors |
|---|---|
| Peppers & chillies | 8–10 weeks |
| Tomatoes | 6–8 weeks |
| Aubergine | 8–10 weeks |
| Brassicas (cabbage, broccoli) | 4–6 weeks |
| Lettuce | 3–4 weeks |
| Courgette & squash | 3–4 weeks |
These are typical ranges, not rules — always check your own seed packet, as varieties differ.
Limitations
- Last frost dates are averages. A late frost is always possible; harden off seedlings and watch the forecast.
- Indoor conditions vary. Light and warmth change how fast plants grow, so treat the date as a target week, not a fixed day.
- It assumes sowing under cover. Outdoor sowing follows a different set of dates.
Frequently asked questions
What is a last frost date?
The average date after which frost is unlikely in spring. It is a probability, not a fixed point, which is why gardeners hedge around it.
What if I sow too early?
Seedlings get leggy and stressed waiting to go out. It is usually better to sow a little late than too early, especially without extra light.
Does this work for outdoor sowing?
Not directly. This tool is for indoor starts; outdoor sowing is usually timed the other way, once the soil has warmed.
Related guides
Sources and research notes
Weeks-before ranges and frost-date logic follow common horticultural guidance; always defer to your seed packet and local weather service.