Grow Light Coverage Calculator
Photon output divided by your area, converted to PPFD and daily light integral, scored against the published bands. The formula and a worked example sit underneath.
By Scooter M. — An enthusiast who's genuinely into this. I read the manuals, compile the published specs, and do the math. No lab coat.
Published · How we pick
Photographs on this site are licensed stock and are illustrative of the fixture type. We do not own the units we write about.
How this page is funded: the buy links here are Amazon Associates links, and we earn a commission on qualifying purchases at no extra cost to you. We have not been paid to feature anything on this page, we accept no free product, and commission plays no part in the ranking. Full disclosure.
We have not tested these units. Everything below comes from the manufacturer’s own published specifications, or from arithmetic we have shown our working for. Where a figure is not published, we say so instead of estimating one. Read the method.
Quick picks
Efficacy is photons out per joule in. Where a manufacturer publishes it we quote theirs; where they publish photon output and wattage but not efficacy, we divide and label it as ours; where they publish neither, the cell says so rather than guessing. Tap a row for the full write-up.
| # | Product | Best for | Efficacy | Price |
|---|---|---|---|---|
| 1 |  Mars Hydro TS 1000 The most photons of anything on this list, and one of only three fixtures here that publishes both PPF and efficacy so the claim can be checked against itself. | 343 µmol/s to work with | 2.3 umol/J | |
| 2 |  Spider Farmer SF1000 Publishes PPF to two decimal places and an efficacy figure that actually divides out of it. That internal consistency is rarer in this category than it should be. | 249.21 µmol/s, published | 2.5 umol/J | |
| 3 |  SANSI 36W BR30 Grow Light Bulb The rare grow bulb that publishes both PPF and a PPFD figure with the measurement distance attached, which is the only reason it can be sized honestly. | 65.6 µmol/s for one plant | 1.82 umol/J (ours) |
The reasoning
Sizing a grow light is one division and one multiplication. This tool does both, scores the answer against the light-level bands published by University of Minnesota Extension, and shows every step so you can check it by hand.
It needs one number you may have to hunt for: PPF, the fixture’s photon output in µmol/s. It is the only figure that can be divided by an area. If your fixture does not publish one — several popular ones do not — skip to the section on what to do about that.
Coverage calculator
From the manufacturer’s spec sheet. If they publish efficacy in umol/J instead, multiply it by the wattage.
0.8 is a reasonable default for a single fixture over a small area. Use 1.0 to see the theoretical ceiling.
Average PPFD at the canopy
738
umol/m²/s
Daily light integral
37.2
mol/m²/day
Above the high-light band
Over 450 umol/m2/s is more than Minnesota Extension's high band. Useful for fruiting crops; wasted on a monstera.
Your arithmetic
343 umol/s × 1 fixture = 343 umol/s.
4.00 sq ft ÷ 10.7639 = 0.372 m².
343 ÷ 0.372 = 923 umol/m²/s at perfect uniformity, × 0.8 = 738.
DLI = 738 × 14 h × 3600 ÷ 1,000,000 = 37.2 mol/m²/day.
The formula, in full
DLI (mol/m²/day) = PPFD × hours × 3600 ÷ 1,000,000
area (m²) = width (ft) × depth (ft) ÷ 10.7639
The 3600 converts hours into seconds; the 1,000,000 converts micromoles into moles; 10.7639 is square feet per square meter. The uniformity factor is the only judgment call in the whole calculation, and it is discussed below.
A full worked example you can re-run
Mars Hydro publishes 343 µmol/s for the TS 1000. A 2 × 4 ft shelf is 8 sq ft, which is 8 ÷ 10.7639 = 0.743 m².
PPFD = 343 ÷ 0.743 × 0.8 = 369 µmol/m²/s. That sits in Minnesota Extension’s high-light band of 250–450.
DLI = 369 × 14 × 3600 ÷ 1,000,000 = 18.6 mol/m²/day.
Seedlings want about 12 mol/m²/day per University of New Hampshire Extension, so this setup is over target. The useful conclusion is not “buy a bigger light” — it is that you could light more shelf with the same fixture, or run it fewer hours and pay less.
About the uniformity factor
A real fixture is brighter directly beneath its center than at the edges of its footprint. Dividing PPF by area assumes perfectly even distribution, which no light achieves, so the result is multiplied by a uniformity factor to approximate an average across the area.
We default to 0.8, and we want to be explicit that this is an assumption rather than a measurement — the one place on this site where the arithmetic is not purely mechanical. It is deliberately conservative. If your fixture is a long bar matched closely to a long shelf, the real figure is likely higher; if it is a small panel hung low over a wide area, it will be worse, and the corners will be much worse than the average implies.
An assumption, not a measurement. Raise it toward 0.9 for a bar closely matched to its shelf; drop it toward 0.6 for a compact panel spread over a wide area.
Scoring your answer
| Band | PPFD (µmol/m²/s) | Typical plants |
|---|---|---|
| Low | 50–150 | Pothos, snake plant, ZZ, most ferns |
| Medium | 150–250 | Seedlings, cuttings, most flowering houseplants |
| High | 250–450 | Succulents, leafy herbs, salad greens, sturdy transplants |
| Above the published bands | 450+ | Fruiting crops; wasted on a foliage houseplant, and you pay for it |
If your fixture does not publish PPF
This is common and it is not a small problem. Barrina publishes no photon figure for any strip in its T5 range; ViparSpectra publishes none for the P1000. You have three honest options:
- Derive it, if efficacy is published. PPF = watts × efficacy. VIVOSUN publishes 100 W and 2.75 µmol/J, so 100 × 2.75 = 275 µmol/s. Mark it in your own notes as derived, because it is arithmetic on their claim rather than their measurement.
- Use a comparable published fixture as a sanity check. Not a substitute for real data, but knowing that similar 100 W panels publish 250–275 µmol/s tells you the order of magnitude.
- Accept that you are buying on form factor. Which is legitimate — it is exactly why we still recommend Barrina strips for shelves — as long as you know that is what you are doing.
What we will not do, and what you should be wary of anywhere else, is publish a made-up PPF for a fixture that does not have one.
What to do with the result
Once you have a PPFD figure, check it against targets by plant type. If you are over, you can light more area or run fewer hours. If you are under, you can hang the fixture lower, run it longer, shrink the area you are covering, or add a second fixture — in roughly that order of cost.
Then price it: the running-cost calculator takes the wattage and the hours you just settled on and turns them into a monthly and a whole-season figure.
Every pick in full
Mars Hydro TS 1000
LED panel343 µmol/s to work with
The most photons of anything on this list, and one of only three fixtures here that publishes both PPF and efficacy so the claim can be checked against itself.
What Mars Hydro publishes
- Power draw
- 150 W +/-5% at 100-277 V AC (source)
- PPF (photon output)
- 343 umol/s (source)
- Efficacy
- 2.3 umol/J (source)
- Stated coverage
- 2.5 x 2.5 ft vegetative, 2.3 x 2.3 ft flowering (source)
- Spectrum
- 660-665 nm, 730-740 nm, 3000 K, 5000 K (source)
- Diodes
- Bridgelux (source)
- Dimming
- Yes, 0-100% dimmer knob (source)
- Dimensions
- 406 x 326 x 50 mm (source)
Our arithmetic
- Average PPFD across its stated 2.5 x 2.5 ft vegetative footprint: about 470 umol/m2/s
2.5 ft x 2.5 ft = 6.25 sq ft = 0.581 m2. 343 umol/s divided by 0.581 m2 = 590 umol/m2/s at perfect uniformity; applying a 0.8 uniformity factor gives about 472 umol/m2/s average. - Electricity used at 14 hours a day for 30 days: 63.0 kWh a month
150 W x 14 h x 30 d / 1000 = 63.0 kWh.
Who should skip it
Skip it if you are lighting a bookshelf. 343 umol/s concentrated into a 2.5 ft square is far more light than a monstera wants, and you pay 150 W to deliver it.
Spider Farmer SF1000
LED panel249.21 µmol/s, published
Publishes PPF to two decimal places and an efficacy figure that actually divides out of it. That internal consistency is rarer in this category than it should be.
What Spider Farmer publishes
- Power draw
- 100 W +/-5% at 100-277 V AC (source)
- PPF (photon output)
- 249.21 umol/s (source)
- Efficacy
- 2.5 umol/J (source)
- Stated coverage
- 2 x 2 ft core, 3 x 3 ft maximum (source)
- Spectrum
- 650-665 nm, 730-740 nm, 2800-3000 K, 4800-5000 K (source)
- Diodes
- Bridgelux (source)
- Dimming
- Yes, dimming knob (source)
- Dimensions
- 32.5 x 29.0 x 5.9 cm (source)
- Rated life
- 55,000 hours (source)
Our arithmetic
- Does the published efficacy check out against the published PPF?: Yes — 2.49 umol/J computed against 2.5 umol/J published
249.21 umol/s / 100 W = 2.4921 umol/J. The published 2.5 umol/J is that number rounded. Not every brand's two figures agree like this, which is exactly why we divide them. - Average PPFD across its 2 x 2 ft core footprint: about 536 umol/m2/s
2 ft x 2 ft = 4 sq ft = 0.372 m2. 249.21 / 0.372 = 670 umol/m2/s at perfect uniformity; at a 0.8 uniformity factor, about 536 umol/m2/s.
Who should skip it
Skip it if raw output is what you are buying — the TS 1000 delivers 38% more photons per second, at the cost of 50 W more draw.
SANSI 36W BR30 Grow Light Bulb
Grow bulb65.6 µmol/s for one plant
The rare grow bulb that publishes both PPF and a PPFD figure with the measurement distance attached, which is the only reason it can be sized honestly.
What SANSI publishes
- Power draw
- 36 W (source)
- PPF (photon output)
- 65.6 umol/s (source)
- PPFD
- 265.58 umol/m2/s at 1 ft (source)
- Spectrum
- 4000 K, 400-780 nm, CRI close to 100 (source)
- Rated life
- 25,000 hours (source)
Our arithmetic
- Efficacy, computed from the published figures: 1.82 umol/J
65.6 umol/s / 36 W = 1.82 umol/J. That is roughly a third less efficient than the panels on this page — the price of a screw-in form factor. - Electricity used at 12 hours a day for 30 days: 12.96 kWh a month
36 W x 12 h x 30 d / 1000 = 12.96 kWh.
What SANSI does not publish
- Beam angle
- Fixture dimensions
Who should skip it
Skip it if you are lighting more than one plant. 65.6 umol/s is a small amount of light spread over a cone — it lights a single specimen, not a shelf.
Questions people actually ask
+What PPF do I need for a 2x2 ft area?
+Why does the calculator ask for a uniformity factor?
+Can I use this with a fixture that only lists watts?
+Is the result the light at the top of my plants or at the soil?
Sources
- University of Minnesota Extension — Lighting for indoor plants and starting seeds — consulted September 2, 2026
- University of New Hampshire Extension — Growing Seedlings Under Lights — consulted September 2, 2026
- Virginia Cooperative Extension — Calculating and Using Daily Light Integral (DLI), SPES-720 — consulted September 2, 2026
- Mars Hydro — TS 1000 published specification table — consulted September 2, 2026
- VIVOSUN — VS1000 product page — consulted September 2, 2026
Read next
Coverage & Sizing
What Size Grow Light Do I Need?
Size from area and target, not from watts. The sum, plus a lookup table by area.
Coverage & Sizing
PPFD Targets by Plant Type
Published light bands mapped to common indoor plants, with the DLI each implies.
Spectrum & PPFD
PAR, PPFD and DLI, Explained
The three photon units, what each measures, and the conversions between them, shown.
Running Cost
Grow Light Electricity Cost Calculator
Watts, hours and your own electricity rate, turned into monthly and seasonal cost.
Where this sits
This page belongs to Coverage and Sizing Math. Every figure on it is either quoted from a named published source or derived in view — the full method is on our methodology page, and how the site is funded is on the affiliate disclosure.