Choose a transplant date after checking your local forecast and crop requirements. Enter the minimum and maximum weeks recommended by your seed supplier. The sowing range is calculated by subtracting those intervals; hardening appears separately and is not added twice. The default numbers illustrate the inputs, not a recommendation for every crop.
Seed-starting & hardening planner
Work backward from transplanting using your variety’s instructions. Hardening is shown separately, not added twice.
These tools calculate from your inputs. Save or print with your browser. Check forecasts, soil conditions and seed-packet instructions before planting.
These calculations use your inputs locally in the browser; they do not infer city frost dates. Read our gardening disclaimer and compare all planting tools.
How to use the seed-starting calculator
- Pick the date you plan to move seedlings outdoors. Base it on your last frost date and on how tender the crop is, not on the first warm afternoon.
- Look at your seed packet or supplier page for the recommended weeks from sowing to transplanting. Most list a range, such as 6 to 8 weeks.
- Enter the minimum and maximum weeks. The calculator counts backward from the transplant date to give an earliest and latest sowing date.
- Add your hardening period if your supplier lists it separately. It is shown on its own so it is not counted twice.
Worked example
Say you plan to transplant tomatoes on May 15 and the packet says 6 to 8 weeks. Eight weeks before May 15 is March 20, and six weeks before is April 3. Your sowing window is March 20 to April 3. Sow early in the window if you want larger transplants and late in the window if your indoor space is limited.
Common mistakes
- Using the packet weeks for direct-sowing crops. Beans, peas and carrots usually do not need to be started indoors.
- Treating the average last frost date as a guarantee. Check the forecast before moving plants outside.
- Forgetting hardening. Seedlings moved straight from indoors to full sun can be damaged even when the air is warm.
For the reasoning behind these timings, read our guide on planning indoor seed starting backward from transplanting and the explainer on when to start seeds indoors.