PPFD is a speed, DLI is a distance
Photosynthetic photon flux density is the number of photons in the 400 to 700 nanometre band landing on a square metre every second, measured in micromoles per square metre per second. It is an instantaneous rate. Daily light integral is what accumulates when that rate runs for a while, measured in moles per square metre per day. The conversion is arithmetic and there is nothing hidden in it:
DLI = PPFD × hours × 3600 ÷ 1,000,000
The division by a million is only the step from micromoles to moles. So 400 PPFD for 16 hours is 400 × 57,600 ÷ 1,000,000, which is 23.04 mol/m²/day. Cut the photoperiod to 12 hours and the same fixture at the same height delivers 17.28. Nothing about the light changed; the total did.
This is why comparing two setups by meter reading alone tells you very little. A modest fixture running long hours and a powerful one running short hours can land on the same DLI, and the plants respond to the total far more than to the peak. It also explains why moving a fixture up to spread the footprint costs intensity but can be recovered with time, up to the point where the species needs a dark period.
Coverage is a fixture number divided by the intensity you want
PPF and PPFD look similar and are not. PPF is the total photon output of the fixture in micromoles per second, a single figure on the spec sheet with no area attached. PPFD is what arrives on a surface. Divide PPF by the area it is spread over and you get the average PPFD, and the same relationship run backwards gives the area a fixture can cover at a chosen intensity.
The complication is that not all of the output lands on plants. Some hits the walls, some hits the floor around the pots, some leaves through the top of an open bench. The capture share in this calculator is that loss. In a small tent with fresh reflective walls and a canopy that fills the footprint, capture is high. On an open shelf in a room with dark walls, a large fraction of what the fixture produces never touches a leaf, and the effective coverage shrinks accordingly.
| Figure | Units | What it describes |
|---|---|---|
| PPF | umol/s | Total photon output of the fixture, area-independent |
| PPFD | umol/m2/s | Photons arriving on one square metre each second |
| DLI | mol/m2/day | Photons accumulated over the whole photoperiod |
| Efficacy | umol/J | Photons produced per joule of electricity drawn |
| Wattage | W | Electricity consumed, which tells you nothing about light |
Why wattage is the worst number on the box
Watts measure what a fixture consumes, not what it emits. Two fixtures at the same draw can differ by more than half in photon output depending on the emitters, the driver and the optics. Efficacy in micromoles per joule is the honest comparison, and it is PPF divided by wall draw. A figure of two and a bit micromoles per joule is common in current LED fixtures; older technology and cheap fixtures sit well below.
The number to be most careful with is any wattage described as equivalent to something else. It is a marketing figure with no measurement behind it. If a spec sheet gives no PPF at all, there is no way to work coverage from it, and there is no substitute for putting a quantum meter under the fixture.
Measuring instead of calculating
Every figure on this page is an average across a footprint, and real footprints are not uniform. Intensity under the centre of a fixture can be double what it is at a corner of the same tent. If you have a meter, take readings on a grid at canopy height, average them, and use that average here rather than the peak. If you do not have a meter, treat the coverage figure as a starting point for where to hang the fixture and expect to adjust.
Height changes everything and changes it fast. Intensity falls roughly with the square of distance from a point source, and a fixture is not a point source, so the real falloff sits somewhere between inverse-square and gentler. Raising a fixture is the cheapest way to trade intensity for evenness, and lowering it is the cheapest way to burn the tops of the plants nearest the middle.
Anything electrical living in a warm, humid room with water in it deserves proper circuit protection and a supply put in by someone licensed to do it. Plugging a fixture into an existing outlet is a homeowner job; adding circuits, running cable or wiring anything into a wall is not, and this page gives no wiring guidance at all.
Where this fits with the rest of the setup
Light drives transpiration, and transpiration is what moves water and dissolved minerals through the plant, so the DLI figure here is upstream of nearly everything else. The VPD calculator covers whether the air conditions let that transpiration happen at all, and the grow tent ventilation calculator covers removing the heat that all of these watts turn into. For general room lighting rather than plant lighting, which is measured in lumens on a completely different curve, use the lighting layout calculator instead, and for the electricity side of a long photoperiod the electricity bill calculator handles the whole-house picture.
Questions people ask
What DLI should I be aiming for?
That depends entirely on the species, the cultivar and the growth stage, and this page deliberately does not answer it. Light requirements vary enormously between plants: shade-tolerant foliage species saturate at levels that would barely register for a fruiting crop, and the same plant wants different totals as a seedling and at maturity. The figure belongs in your own crop notes or a horticultural reference for what you are actually growing. What this calculator does is convert between the numbers, so that once you have a target from a credible source you can work out whether your fixture, height and photoperiod deliver it.
Why does my meter reading not match the coverage this gives?
Almost always because the reading is from one spot and the calculation is an average. A fixture concentrates light under its middle and drops off toward the edges, so a centre reading overstates the footprint average by a wide margin, sometimes by a factor of two. Take readings on a grid across the canopy and average them. The other common cause is the capture share: the default assumes a reflective enclosure with the canopy filling the footprint. On an open bench, or with a small plant under a large fixture, a great deal of the output never lands on a leaf, and lowering the capture figure brings the numbers back into line.
Is PPF the same as PPFD?
No, and the two get swapped constantly in product listings. PPF is the fixture output in micromoles per second with no area attached, a property of the light itself. PPFD is what arrives at a surface, in micromoles per square metre per second, and it depends on distance, footprint and what the light bounces off. A listing that quotes a PPFD number without saying at what distance and at what point in the footprint is quoting nothing useful. If only PPFD is given, ask for PPF, because that is the figure coverage can be worked out from.
Does running the light longer at lower intensity give the same result?
For the daily total, yes, and that is exactly what the DLI arithmetic says. Biologically it is not equivalent. Many species use day length as a developmental signal quite separately from how much light they receive, so changing the photoperiod can change what the plant does rather than just how fast it grows. Some plants also need a genuine dark period for normal function. Treat DLI as the accounting for energy captured, and photoperiod as a separate decision driven by the species. This calculator will happily show you that 24 hours at low intensity matches 12 at high, and whether that is a sensible thing to do is a horticultural question.
How do I convert lux or foot-candles to PPFD?
Not reliably, and this calculator does not try. Lux and foot-candles are weighted by the sensitivity of the human eye, which peaks in green and falls away sharply in blue and deep red — exactly the regions plants use heavily. The conversion factor between lux and PPFD therefore depends on the spectrum of the source, and it differs by a large margin between daylight, a warm white LED and a fixture with heavy red emitters. Any single conversion number you find is valid only for the specific spectrum it was derived from. A quantum sensor measures photons across the whole band directly, which is why it is the tool for the job.