How many grow lights does your seed starting shelf need?
Enter your shelf size and fixture length. You get the number of lights, how many seed trays fit, and what a season of running them costs. Hanging height and hours come from university extension sources, and we show where they disagree.
What extension sources agree on, and where they don't
Most seed starting advice online repeats one confident number. University sources give ranges, and the ranges depend on the kind of light.
| Question | Common answer | Where sources differ |
|---|---|---|
| Hours a day | 14 to 16 hours for seedlings (Iowa State, UConn, Maryland, Oklahoma State) | One Iowa State answer about a standard fluorescent shop fixture says 12 to 14 hours. UConn says greens and herbs can do with about 12. |
| Distance above seedlings | Close, then raise the light as plants grow | Fluorescent: 1 to 2 inches (Maryland), 4 to 6 inches (Iowa State), 6 to 8 inches (Oklahoma State). LED: about 8 to 12 inches (Iowa State). Follow your fixture's manual. |
| Special grow light needed? | Usually not | Iowa State says plain full-spectrum shop lights are fine for most home gardeners. Mississippi State agrees. |
| Lights per shelf | Two 4-foot fixtures over a 24 x 48 inch shelf | That is the University of Saskatchewan's classroom example. Fruiting plants grown to maturity may want up to four. |
Sources are linked in the guide.
How the shelf calculator works
You type four things: shelf length, shelf depth, fixture length and the watts printed on the fixture. The calculator then does simple arithmetic you can check by hand.
- Fixtures. One row of lights covers about 12 inches of shelf depth (our planning rule, not a standard, and you can change it). A 24 inch deep shelf gets two rows. Each row needs enough bars to span the shelf length, with a 5 percent tolerance before another bar is added.
- Trays. It tries a tray both ways round and counts whole trays only. The default is a 1020 flat, about 21 x 11 inches on the outside. Measure yours, because trays vary.
- Running cost. Watts x hours per day x days, divided by 1,000, gives kWh. Multiply by your electricity price. The default is 14 hours a day for 70 days, which you can change.
Worked example. A 24 x 48 inch shelf with 4-foot, 40 watt fixtures needs 2 fixtures in 2 rows. That is 80 watts, and four 1020 flats fit. At 14 hours a day for 70 days that is 78.4 kWh, or about $11.76 at 15 cents per kWh. With 2-foot, 10 watt bars the same shelf needs 4 bars and draws 40 watts in total.
Try it with your own shelf. The address updates as you type, so you can bookmark a setup.
What to check before you buy a light
Most of these points come from university extension pages, linked in the guide. The last one is general shopping advice.
- Brightness, not the word "grow". Iowa State suggests a full-spectrum LED of at least about 3,000 lumens. Mississippi State says shop lights are fine at roughly 2,000 to 3,000 lumens or more, as long as they hang close to the plants.
- Fixture length against shelf length. Oklahoma State notes that the ends of a fluorescent tube lose about 3 inches of intensity. A bar the same length as the shelf leaves the corners dimmer.
- Distance matters as much as watts. The University of New Hampshire gives an LED bar example of 8 hours a day at 8 inches or 16 hours at 20 inches. Light fades quickly with distance, so fixtures need to be adjustable in height.
- A timer. Most sources want 14 to 16 hours of light a day and a dark period. A plain timer is easier than remembering.
- What the listing really says. Look for watts, lumens and the length of the fixture. If a listing gives only a big "equivalent" wattage, treat that number with caution.
Common questions
How many grow lights do I need for one shelf?
For a 24 x 48 inch shelf the University of Saskatchewan shows two 4-foot fixtures, and up to four for fruiting plants grown to maturity. Use the calculator for other sizes.
Do I need a special grow light for seedlings?
Not always. Iowa State says grow lights are often unnecessary and that full-spectrum shop lights work for most home gardeners. Brightness and distance matter more than the label.
How many hours a day should the lights be on?
Iowa State, UConn, Maryland and Oklahoma State give 14 to 16 hours. Iowa State also has a 12 to 14 hour answer for a fluorescent shop fixture, so check the guide if you use one.
How far above the seedlings should the light be?
It depends on the light. Sources give 1 to 2 inches, 4 to 6 inches or 6 to 8 inches for fluorescent tubes, and about 8 to 12 inches for LED. Follow your fixture's manual and watch the plants.
Are your product picks tested?
No. Pages here are researched from extension sources and manufacturer specifications, not hands-on testing, and they say so. See how we pick products.
| Shelf light calculator | Fixtures, trays, power and running cost for your shelf. |
| Shelf lighting guide | Sizing, hanging height, hours and the sources behind them. |
| All guides | Hanging height, hours, shop lights, 4-foot shelves, heat mats, fans, running cost, seed starting basics and crop timing. |
| Frost-date calculator | When to start seeds indoors for your last frost date. |
| Gear shortlist | Light bars and a heat mat, with the specifications manufacturers publish. |
| How we pick products | What we check on a listing and what we do not claim. |
| Affiliate disclosure | Currently no affiliate links. Any future link will be marked. |
Start here
If you already know your shelf size, use the calculator. If you are still deciding what to buy, the guide covers sizing, hanging height, timers and what to check on a product listing.
This site does not sell anything. Product lists will be added over time, and any affiliate link will be marked. See how we pick products and the affiliate disclosure.