Tomato seedlings need bright light very close overhead, or they stretch tall and weak reaching for it. How close depends on the light: fluorescent tubes sit a few inches away, while strong LED grow lights hang higher. Pick your light and stage to see how high to hang it and how long to run it.
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What Your Result Means: the height is measured from the light to the top of the seedlings, not the soil. Keep it in that range and raise the light as the plants grow. Weak, floppy seedlings almost always mean the light is too far away or runs too few hours.
How High Should a Grow Light Be?
It depends on the light. Fluorescent tubes go just a few inches above the seedlings, while a strong LED grow light hangs a foot or two higher. Pick your light type and the seedling stage, and the tool gives a safe hanging height and how many hours a day to run it.
- Choose Your Light Type, the fixture over your seed tray.
- Choose the Stage, from just sown to seedlings with true leaves.
- Set the Height, measured to the tops of the seedlings, and raise it as they grow.
What Each Result Means
| Result | What It Tells You |
|---|---|
| Hanging height | Distance from the light to the seedling tops |
| Hours per day | How long to run the light |
| As they grow | When to raise the light |
Height by Light Type
Typical hanging heights above the tops of tomato seedlings. Follow the maker’s guidance for a specific grow light.
| Light Type | Height Above Tops | Hours per Day |
|---|---|---|
| Fluorescent (T5 or T8) | 2 to 4 inches | 14 to 16 |
| Compact fluorescent (CFL) | 3 to 6 inches | 14 to 16 |
| LED shop light | 6 to 12 inches | 14 to 16 |
| Full-spectrum LED grow light | 12 to 24 inches | 14 to 16 |
| Sunny window only | No light to hang | Expect leggy seedlings |
How Does It Work?
Different lights put out different intensity, so each has a safe distance where seedlings get plenty of light without heat stress. The tool matches your light type to that range and reminds you to raise it as the plants grow.
LED shop light: 6 to 12 inches · LED grow light: 12 to 24 inches
Run 14 to 16 hours a day for seedlings
Raise the light as plants grow to keep the gap
Weak fluorescent and shop lights must sit close because their light fades fast with distance. Strong grow lights are hung higher so they do not scorch the leaves.
Worked Example
Say you use a fluorescent T5 over seedlings with their first leaves. The tool tells you to keep the tubes 2 to 4 inches above the tops and run them 14 to 16 hours a day, raising the light every few days so the small gap stays the same as the seedlings grow.
Factors That Change the Height
- Light strength: a stronger fixture hangs higher; a weak one must sit close.
- Heat: if leaves feel warm under the light, raise it; LEDs run cooler than old bulbs.
- Seedling stretch: tall, leaning seedlings are reaching for light, so lower it or add hours.
- Coverage: seedlings at the edge of the light get less, so center the tray.
- Day length: seedlings do best with 14 to 16 hours, then a dark rest period.
Understanding Your Outputs
| Output | Meaning | Best Used For |
|---|---|---|
| Hanging height | Gap from light to tops | Setting up the light |
| Hours per day | Run time on a timer | Avoiding leggy seedlings |
| As they grow | When to raise it | Keeping the gap right |
When to Use It
Use it when setting up lights for tomato seedlings indoors, or when seedlings turn leggy and you suspect the light is too far or too dim. Pair it with the germination calculator for timing and the hardening-off scheduler before transplanting.
Common Mistakes to Avoid
- Light too far: the top cause of leggy seedlings; keep fluorescents just above the tops.
- Measuring to the soil: measure the gap to the seedling tops, not the tray.
- Forgetting to raise it: the gap grows as seedlings do, so adjust every few days.
- Too few hours: a sunny window is rarely enough; run lights 14 to 16 hours.
- No dark period: seedlings still need several hours of darkness, so avoid running lights all day and night.
Accuracy and Limitations
These are typical ranges for common seed-starting lights.
What It Gives Reliably: a safe starting height and run time by light type for tomato seedlings.
What It Does Not Cover: the exact output of your specific fixture. Strong grow lights vary widely, so follow the maker’s distance chart and watch how your seedlings respond.
Methodology
Method: your light type is matched to a typical hanging height and run time for seedlings, with a note for the stage. Inputs Used: light type and seedling stage. Ranges: fluorescent tube 2 to 4 inches, CFL 3 to 6 inches, LED shop light 6 to 12 inches, full-spectrum LED grow light 12 to 24 inches, all run 14 to 16 hours a day for seedlings. Guidance: measure to the seedling tops and raise the light as plants grow. Note: grow-light output varies, so follow the maker’s guidance and watch for stretch or heat.
Sources: Iowa State University Extension – Growing Tomatoes, The Old Farmer’s Almanac – Tomatoes, Royal Horticultural Society – Tomatoes. Last reviewed: September 25, 2026 · Reviewed by Aiza Anwar.
Frequently Asked Questions
How high should a grow light be above tomato seedlings?
It depends on the light. Keep fluorescent tubes 2 to 4 inches above the tops, LED shop lights 6 to 12 inches, and full-spectrum LED grow lights 12 to 24 inches, adjusting as the seedlings grow.
How many hours of light do tomato seedlings need?
Run grow lights 14 to 16 hours a day, then give seedlings several hours of darkness. A sunny window alone is usually not enough and leads to leggy plants.
Why are my tomato seedlings tall and leggy?
Almost always the light is too far away or runs too few hours, so seedlings stretch to reach it. Lower the light close to the tops and run it 14 to 16 hours a day.
Can I grow tomato seedlings on a windowsill?
You can, but a window rarely gives enough light in late winter, so seedlings often turn leggy. A simple fluorescent or LED light kept close gives much sturdier plants.
Do tomato seeds need light to germinate?
No. Seeds sprout in the dark under a light covering of mix. Turn the light on the moment seedlings break the surface, and keep it close from then on.
Are LED or fluorescent lights better for seedlings?
Both work well. LEDs run cooler and use less power, while fluorescent tubes are cheap and effective. The key is intensity and keeping the light at the right distance.
Should I leave grow lights on all night?
No. Seedlings need a daily dark period, so run lights about 14 to 16 hours and off the rest. A timer makes this easy and consistent.
How do I know if the light is too close?
If the leaves feel warm under the light or look bleached or curled, raise the light. LEDs and fluorescents run cool, so this is more likely with older or high-heat bulbs.
How often should I raise the grow light?
Check every few days and raise the light as the seedlings grow so the gap stays in the recommended range. Fast-growing tomatoes may need adjusting twice a week.
Related Tomato Tools
Time It with the Germination Calculator
This tool gives typical grow-light heights for common seed-starting fixtures. Grow-light output varies, so follow your light’s guidance and watch how seedlings respond. Measure the gap to the seedling tops and raise the light as they grow. Spotted an error? Tell us here.