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ToolEntry 42Reviewed 2026-10-16

Seed starting calculator: count back from transplant day

Pick the day your seedlings should go outside. The calculator counts back UNH Extension's weeks from seed to transplant, adds Iowa State's days to germination and marks the two-week hardening-off start that Minnesota Extension recommends. Montana State's and Iowa State's own week counts are shown for comparison.

Seed-starting schedules work backward. UNH Extension's planting fact sheet gives each crop a number of weeks from sowing until transplant, and a UNH Extension Master Gardener answer shows the count: to transplant on May 27, sow seeds that need 6 to 8 weeks between April 1 and April 15. This calculator does that count for 19 crops started indoors, then adds what the weeks alone do not tell you: when the first seedlings should appear and when to start hardening them off. If you do not have a transplant date yet, let the calculator set one from your average last spring frost using UNH's planting-out offsets. Finding that frost date is covered in frost dates explained; mix, lights and care between sowing and transplant are covered in seed starting indoors.

The example date is UNH's May 27 transplant date; replace it with your own.

CropTransplantSow indoors (UNH weeks)Seedlings up (Iowa State days)Start hardening off (UMN)Other sow windows
The formula. With a transplant date T: sow indoors from T − (most weeks × 7 days) to T − (fewest weeks × 7 days), using UNH's weeks from seed to transplant. Seedlings up = earliest sow date + fewest germination days, to latest sow date + most germination days (Iowa State). Start hardening off = T − 14 days (UMN: two weeks before planting outdoors). From a frost date F, the transplant window is first set to F + UNH's planting-out offset in weeks (negative = before the frost), and the same steps follow. "Other sow windows" repeat the first step with Iowa State's weeks from sowing to planting and Montana State's weeks to transplant size. Nothing is saved or sent; the page does all its work in your browser.

Worked examples

  1. Tomato, transplant May 27. UNH gives 6–8 weeks from seed to transplant. May 27 minus 56 days is April 1; minus 42 days is April 15, the same window UNH's Master Gardener answer gives. Iowa State lists 6–12 days to germination for tomato at 70–75°F, so seedlings should appear between April 7 (April 1 + 6) and April 27 (April 15 + 12). Hardening off starts May 13.
  2. Parsley, last frost May 1. UNH sets parsley out 2–3 weeks before the last frost, so the transplant window is April 10 to April 17. With 9–10 weeks indoors, sowing runs from January 30 (April 10 minus 70 days) to February 13 (April 17 minus 63 days). Iowa State's 20–25 days to germination put the first seedlings between February 19 and March 10, so a tray sown on January 30 may show nothing until February 19.
Counting back from a transplant date: tomato exampleA timeline from April 1 to May 27. A seed marker at April 1 is followed by a short germination band from April 7 to April 13, a long seedling-growth band, a two-week hardening-off band starting May 13 and a transplant marker at May 27. An arrow along the bottom points from the transplant date back to the sowing date, labelled 8 weeks. Numbered markers match the list below the drawing. seedlings under lights harden off Sow Apr 1 up Apr 7–13 May 13 Transplant May 27 count back 8 weeks (56 days) 1 2 3 4 5
Drawing by The More One Sows, based on the cited sources. Spacing is roughly proportional to days; the dates follow the tomato example in the text.
  • 1. Sow: May 27 minus 8 weeks (the long end of UNH's 6–8 weeks for tomato) is April 1
  • 2. Germination: 6–12 days at 70–75°F for tomato (Iowa State), so seedlings appear April 7–13
  • 3. Seedlings grow under lights; these days are already inside UNH's 8 weeks
  • 4. Hardening off starts two weeks before transplanting (UMN): May 13
  • 5. Transplant: May 27, the date in UNH's worked example

Germination days are inside the weeks

UNH counts its weeks "from sowing until transplant", and Iowa State's column is "weeks sowing to planting". Both start on the day the seed goes in, so the days to germination are already part of the count. Adding them on top would start every crop too early. The calculator uses the germination days for a different job: they tell you when to expect the first seedlings, and so when a tray that shows nothing deserves a closer look.

The gap matters most for slow crops. Parsley's 20–25 days to germination take up about three of its 9–10 weeks indoors; celery's 14–21 days come out of 10–12 weeks. Cabbage-family crops come up in 4–8 days, and Iowa State gives cucumbers and melons 3–10 days.

Iowa State's day counts go with its listed germination temperatures, given as ideal air temperatures in °F. UNH's seed-starting fact sheet calls 65 to 75°F best for most plants, while noting that tomato seed will germinate anywhere between 50 and 95°F. Minnesota Extension warns that potting mix can be as much as 5°F cooler than the room air, and that a seed-starting heat mat should not be plugged into the light timer. Iowa State also says which seeds to cover: most are covered with mix, lettuce needs light to germinate and is pressed into the mix without being covered, and onion and celery are covered only lightly.

The figures used

"Weeks indoors" and "plant out" are from UNH's planting fact sheet. Germination days, temperature, covering and the second week count are from Iowa State. The last column is Montana State's "weeks to transplant size". A dash means the source gives no figure for that crop: Montana State has no rows for basil or parsley and treats kale as direct-seeded; Iowa State lists kohlrabi and lettuce only among crops usually sown directly in the garden.

CropWeeks indoors (UNH)Plant out vs last frost (UNH)Days to germinate (Iowa State)Germination temp, °F (Iowa State)Seed covering (Iowa State)Weeks sowing to planting (Iowa State)Weeks to transplant size (Montana State)
Tomato6–81–2 wk after6–1270–75Cover seed5–76–10
Pepper8–102 wk after7–1070–75Cover seed6–86–8
Eggplant8–102–3 wk after7–1070–75Cover seed6–86–10
Basil6–82 wk after10–1465–70Cover seed5–6—
Parsley9–102–3 wk before20–2570–75Cover seed6–8—
Celery10–12After last frost (no week count)14–2170–75Cover lightly10–1210
Onion8–104 wk before4–2070–75Cover lightly8–104–8
Broccoli4–62–4 wk before4–765–70Cover seed4–66–8
Brussels sprouts4–62–4 wk before5–865–70Cover seed4–66–8
Cabbage4–64 wk before4–765–70Cover seed4–66–8
Cauliflower4–62 wk before4–765–70Cover seed4–64–5
Kale4–64 wk before4–765–70Cover seed4–6—
Kohlrabi4–64 wk before4–765–70Cover seed—6–8
Lettuce4–53–4 wk before7–1460–70Press in, do not cover (needs light)—4–6
Cucumber3–41–2 wk after3–1075–80Cover seed3–44
Muskmelon3–42 wk after3–1075–80Cover seed3–43–4
Summer squash, zucchini3–42 wk after5–1075–80Cover seed3–44
Winter squash3–42 wk after5–1075–80Cover seed3–43–4
Pumpkin3–42 wk after5–1075–80Cover seed3–44

UNH lists "melon"; Iowa State and Montana State list muskmelon. Iowa State gives one row for squash and pumpkin. UNH gives celery no week count for planting out, only "after" the last frost, so in frost-date mode the calculator counts celery from the frost date itself; treat that as the earliest date.

Where the sources differ

CropWeeks before transplant, by source
Tomato6–8 (UNH fact sheet); 5–7 (Iowa State); 6–10 (Montana State). UNH's Master Gardener answer phrases it as six to eight weeks before the last frost, while the fact sheet plants tomatoes out 1–2 weeks after it.
Pepper, eggplant8–10 (UNH fact sheet); at least 10 weeks indoors (UNH Master Gardener answer); 6–8 (Iowa State); 6–8 for pepper and 6–10 for eggplant (Montana State)
Onion8–10 (UNH, Iowa State) vs 4–8 (Montana State)
Broccoli, cabbage, Brussels sprouts4–6 (UNH, Iowa State) vs 6–8 (Montana State)
Cucumber3–4 (UNH fact sheet, Iowa State); 4 (Montana State); four to six weeks before the planting date (UNH Master Gardener answer)
Kale, lettuceStarted indoors by UNH (4–6 and 4–5 weeks); Montana State direct-seeds kale and gives lettuce 4–6 weeks; Iowa State lists lettuce with crops usually sown directly

Where the ranges disagree, the "Other sow windows" column shows how much the sowing date moves. Montana State's guide asks you to calculate both ends of a range, which gives a window rather than a single day.

Before you rely on the dates

Crop pages carry each crop's full outdoor timing, for example tomatoes, peppers and parsley.

Sources

  1. University of New Hampshire Extension — Growing Vegetables: When to Plant Your Vegetable Garden (fact sheet)
  2. University of New Hampshire Extension — How do I know when to start seedlings indoors? (S. Walker, March 2018)
  3. University of New Hampshire Extension — Starting Plants Indoors from Seed (fact sheet, B. Sideman, rev. April 2017, PDF)
  4. Iowa State University Extension and Outreach, Yard and Garden — Germination Requirements for Annuals and Vegetables (R. Jauron, A. Steil, reviewed December 2023)
  5. Montana State University Extension — Can I Grow That Here? Vegetable Seed and Transplant Schedules (MontGuide)
  6. University of Minnesota Extension — Starting seeds indoors

All sources were opened and checked in October 2026. Rules and figures change; confirm with the original before acting.

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