Calculate when to start seeds indoors

Count backward from a local last-frost estimate to plan indoor sowing and hardening-off dates.

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ToolSeed Starting Calculator
Input
Output
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How it works

Whole packet weeks are subtracted from the selected frost date for indoor sowing, and the chosen acclimatisation days are subtracted for hardening off. The frost date is displayed as the earliest transplant reference, while soil temperature and forecast conditions remain separate decisions.

Formula
indoor sow date = last-frost date − packet weeks × 7 days
  • RHS advises 10–14 days for hardening, but the exact chosen duration is entered rather than hidden.
  • Last frost and packet lead time have no universal defaults.

Worked example

Tomatoes, 6 weeks before frost
Last frost expected 2026-05-10
Input
											Last frost date: 2026-05-10
Weeks before frost: 6
Hardening days: 10
Output
												Sow indoors by: 2026-03-29
Harden off from: 2026-04-30
Transplant after: 2026-05-10

When to use this

Tomato packets count indoor weeks backward, tender annuals acclimatise outdoors before planting, and succession plans compare cultivars with different lead times.

Edge cases

  • A regional average last frost is not a forecast for one garden; the exact date entered remains only a planning reference.
  • Selecting fewer than the RHS’s 10–14 hardening days still calculates a date but does not turn that shorter choice into RHS guidance.
  • A frost-free date can still have cold soil, wind, or saturated ground, so outdoor planting may need to wait.

References