Seed Starting Calculator: When Should You Sow Indoors?
Sowing too early gives leggy seedlings, and too late wastes the season. Counting back from the last frost finds the right week.
What is the Seed Starting Calculator?
Starting seeds indoors means counting backwards from the last frost. Enter the last frost date and how many weeks early the seeds need, and this tool gives the sowing date.
How to use it
- Enter your values in the fields provided.
- Click Calculate Sowing Date.
- Read the result and adjust an input if you want a different outcome.
Worked example
With a last frost on 2026-05-15, seeds started 6 weeks early go in on 2026-04-03.
When do I start seeds 6 weeks before a 15 May frost?
Six weeks is 42 days, and 42 days before 15 May 2026 is 3 April 2026.
Frequently asked questions
What do I enter to get a result?
Fill in every field with your own numbers or choices, then run the tool. Empty number fields count as zero, so check them before you trust the output.
Can I reuse this result elsewhere?
Yes. The result is calculated in your browser from the values you entered, so you can copy the numbers into your own notes or plan.