Seed-Starting Calculator

How to use this calculator

Enter your local spring frost-free reference, indoor or direct sowing method, and the outdoor planting offset in days. For indoor starts, enter minimum and maximum weeks from sowing to transplant. Add germination and hardening durations from your variety's guidance.

Indoor sowing runs from transplant date minus maximum weeks to transplant date minus minimum weeks. Germination is part of the seedling period, not extra time added to it. The emergence window combines the earliest sowing/minimum germination and latest sowing/maximum germination. Direct sowing uses the outdoor date without indoor or hardening stages.

A worked example

With an April 15, 2027 reference, planting seven days later and a six-to-eight-week indoor period gives a February 25–March 11 sowing window. These are illustrative inputs, not a recommendation for a specific crop.

Can a frost date guarantee that planting is safe?

No. Frost estimates, weather, soil temperature, crop and variety all matter. Use local guidance and your seed packet. The calculator does not infer frost dates, crop timings or location.

Further reading: Johnny's seed-starting guidance. Your entered rules determine this result.

Which calendar and years are supported?

Use Gregorian dates from 1900 through 2100. Each tool shows its own range and counting rules. Calculations run in your browser when requested, with no background polling. Today buttons use the server's current date in your browser's time zone. Calendar downloads contain all-day planning markers; review them before importing.

What happens to my dates?

Calculations run on your device. Saving a calculation is optional and keeps it in this browser. A setup link contains the dates and settings you entered; anyone with the link can read them. Use one tab per saved calculator. Clear saved data from Privacy & settings.