Start Seeds
—
Transplant
—
Trays Needed
0
| Item | Value |
|---|---|
| Plant Type | Custom |
| Last Frost | — |
| Start Indoors | — |
| Transplant Outdoors | — |
| Germination | 7 days |
| Soil Volume | 0 cu ft |
| Trays | 0 |
| Light Hours | 16 hrs |
Seed Starting Calculator: When to Start Seeds Indoors in 2025
Never miss a planting window again. Our seed starting calculator uses your last frost date to give you exact indoor start dates, transplant timing, soil volume, trays, and grow light needs.
How the Seed Starting Calculator Works
Formula:
- Start Date = Last Frost − (Weeks Before Frost × 7)
- Transplant Date = Last Frost + Hardening Days
- Trays = Ceiling(Plants Needed ÷ Cells per Tray)
- Soil = Plants × Cell Volume (cu in) ÷ 1728
Example: Tomato, Last Frost May 15, 72-cell tray, 24 plants
- Start: May 15 − 6 weeks = April 3
- Transplant: May 15 + 7 days = May 22
- Trays: 24 ÷ 72 = 1 tray
- Soil: 24 × 3.5 cu in = 84 cu in → 0.05 cu ft
Seed Starting Timeline by Plant
| Plant | Weeks Before Frost | Transplant After |
|---|---|---|
| Tomato | 6–8 | Last frost |
| Pepper | 8–10 | Last frost |
| Broccoli | 4–6 | 2 weeks before |
| Lettuce | 3–4 | 2 weeks before |
| Cucumber | 2–3 | 1 week after |
| Basil | 6–8 | Last frost |
| Marigold | 4–6 | Last frost |
Soil Volume per Cell
| Tray | Cell Volume | Soil per 100 cells |
|---|---|---|
| 72-cell | 3.5 cu in | 0.20 cu ft |
| 50-cell | 5.0 cu in | 0.29 cu ft |
| 32-cell | 8.0 cu in | 0.46 cu ft |
| 18-cell | 14 cu in | 0.81 cu ft |
Germination Requirements
- Tomato: 75–85°F, 5–10 days
- Pepper: 80–90°F, 7–14 days
- Lettuce: 60–70°F, 7–10 days
- Cucumber: 75–85°F, 3–7 days
Grow Light Guide
- Seedlings: 14–16 hours/day
- Distance: 2–4" above plants
- Type: Full-spectrum LED or fluorescent
- Watts: 20–40W per sq ft
Pro Tips for Success
- Use sterile seed-starting mix
- Pre-moisten soil before filling
- Cover with dome for humidity
- Bottom water to avoid damping-off
- Harden off 7–10 days before transplant
- Label everything!
Common Mistakes
- Starting too early (leggy plants)
- Not enough light
- Overwatering
- Planting too deep
- No air circulation
Supplies Checklist
- Seed trays with dome
- Seed-starting mix
- Heat mat (for peppers/tomatoes)
- Grow lights + timer
- Watering can with rose
- Labels and marker
Hardening Off Schedule
- Day 1–2: 1–2 hours shade
- Day 3–4: 3–4 hours partial sun
- Day 5–7: Full day outside
- Transplant on cloudy day
Related Calculators
FAQs
When to start tomato seeds indoors? 6–8 weeks before last frost.
Can I use garden soil? No — use sterile seed mix.
How deep to plant seeds? 2× seed diameter.
Do all seeds need light to germinate? No — check packet.
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