Seed Starting Date Calculator

Work out seed starting date instantly with clear inputs, formula shown and shareable results.

About Seed Starting Date Calculator

The sowing date counts back from the last frost date by the recommended indoor growing weeks, and hardening off starts a set number of days before planting out. Sowing too early produces leggy seedlings that cannot be planted out, which is why counting back from the frost date beats sowing on a fixed calendar day.

Formula

Sowing date

sow date = last frost date - weeks indoors x 7 days

Worked example

With the default values below, Seed Starting Date Calculator returns Sow indoors on of 2026-03-29. Change any input above and every figure here is recalculated in your browser.

Inputs used

Expected last frost date: 2026-05-10
Weeks to start indoors before that dateminimum 0.5: 6
Days to harden off before planting outminimum 0: 10

Results

Sow indoors on: 2026-03-29
Begin hardening off on: 2026-04-30
Plant out on or after: 2026-05-10
Days growing indoors: 42

Step by step

1Values used
2Sowing date
3Sow indoors on= 2026-03-29
4Begin hardening off on= 2026-04-30
5Plant out on or after= 2026-05-10
6Days growing indoors= 42

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Seed Starting Date figure worked out?

sow date = last frost date - weeks indoors x 7 days. The sowing date counts back from the last frost date by the recommended indoor growing weeks, and hardening off starts a set number of days before planting out.

Why does this matter?

Sowing too early produces leggy seedlings that cannot be planted out, which is why counting back from the frost date beats sowing on a fixed calendar day.

What do I need to enter?

Enter expected last frost date, weeks to start indoors before that date, days to harden off before planting out. The defaults shown are a realistic worked example — swap in your own figures to get a result you can use.