PPFD Grow Light Calculator UK
Step-by-step for flat grows and low-light rooms. Enter your light and space — PPFD and DLI in seconds.
100W × 2.2 µmol/J ÷ 0.6m² = 367 µmol/m²/s
Estimates assume uniform canopy coverage at a fixed height. PPFD falls with the inverse-square law as distance increases — measure at canopy level for best accuracy.
Convert a lux meter reading to approximate PPFD using a light-specific K factor.
20,000 lx × 0.0143 = 286 µmol/m²/s
K factors are approximate. A quantum (PAR) sensor measures plant-relevant photons directly and is more accurate. K factors sourced from University of Maryland Extension and peer-reviewed photobiology literature.
Step-by-step: how to find your PPFD
The calculation is four steps and one multiplication. Get the three inputs right and the answer is exact — there is no guesswork in it once you have real numbers.
Prefer to skip the arithmetic? The calculator above handles both steps live as you type. The DLI output matters for UK winter grows in particular — a UK south-facing windowsill receives under 2 mol/m²/day in January, so your grow light is providing nearly all of a plant's daily photon budget.
PPFD targets by plant type: UK indoor data
Target PPFD is set by each plant's light-saturation point — the level beyond which adding more light yields no extra growth, and above which you risk photoinhibition. These ranges reflect converging horticultural sources and are consistent with the verified DLI data used by commercial UK indoor growers.
| Plant | Target PPFD (µmol/m²/s) | Target DLI (mol/m²/d) | Typical photoperiod |
|---|---|---|---|
| Snake plant (Sansevieria) | 50–120 | 4–8 | 12–14 h |
| Monstera / cheese plant | 100–200 | 6–12 | 12–16 h |
| Phalaenopsis orchid | 75–150 | 5–9 | 12–14 h |
| Indoor lettuce | 150–300 | 12–17 | 16–18 h |
| Basil / culinary herbs | 200–400 | 12–16 | 14–16 h |
| Microgreens | 150–250 | 6–12 | 12–16 h |
| Tomato / fruiting vegetables | 400–600 | 20–30 | 16–18 h |
DLI targets consistent with converging horticultural sources and AHDB Horticulture commercial lighting recommendations. Orchid PPFD range aligned with guidance from the Orchid Society of Great Britain.
Why UK flat grows demand accurate PPFD data
Most PPFD guides are written for the US or for commercial glasshouses. The UK context is different in three important ways.
Winter light deficit. A south-facing UK windowsill delivers roughly 1–4 mol/m²/day DLI in January — compared to 8–12 mol/m²/day needed for lettuce. Supplemental lighting isn't a luxury; it provides the majority of a plant's photon budget from October through March. Getting the PPFD figure right means knowing you've closed that gap.
Low-light houseplants tolerate less than you think. Monstera (cheese plant) thrives at temperatures comfortable for humans — 12–30°C — and performs well at just 100–200 µmol/m²/s. According to Royal Horticultural Society guidance, keep it at least 1.5 m from an operating radiator to avoid leaf scorch from dry heat, not light overload. If a variegated Monstera loses its patterning and reverts to plain green, that's a reliable sign it's not getting enough light — the plant sacrifices variegation for more chlorophyll when photons are scarce.
Snake plants are genuinely low-light. Sansevieria needs only 50–120 µmol/m²/s. Watering requirements also drop in winter — every 2–6 weeks depending on season — and its main vulnerability is overwatering rather than light levels. A single modest LED on a shelf can easily over-deliver light for these plants; 50–75 µmol/m²/s with a 12-hour photoperiod is ample.
Microgreens and herbs bridge the gap. Penn State Extension research on microgreens confirms a productive window of 150–250 µmol/m²/s for most species — achievable from a modest 40–60W quality LED over a standard seed tray. This is exactly the scale that works in a UK kitchen or spare-room shelf system.
PPFD vs PAR vs Lux: which metric matters for UK indoor growing
PPFD measures the intensity of photosynthetically active light at a specific point. PAR defines the waveband (400–700 nm). DLI accumulates both over a day. Lux and lumens measure human-perceived brightness and are irrelevant to plant physiology — understanding which metric to use prevents the most common UK grow-light buying mistake.
µmol/m²/s at canopy; mol/m²/day accumulated. The metrics that drive plant growth decisions — the only numbers that matter for matching light to plant.
The waveband (400–700 nm). Defines what counts as plant-usable light, but doesn't tell you how much is present. PPFD is PAR measured at a point.
Human eye brightness. Green-heavy lights score high in lumens but deliver poor plant photons. Red-blue LEDs look dim to our eyes but can deliver excellent PPFD.
| Metric | Unit | What it measures | Plant relevance |
|---|---|---|---|
| PPF | µmol/s | Total photon output of the fixture | Good — use to compare fixtures at a fixed size |
| PPFD | µmol/m²/s | Photon density at a specific point | Best — directly comparable to target ranges |
| DLI | mol/m²/d | Total daily photons received | Best — accounts for photoperiod |
| Lux (lx) | lux | Brightness to the human eye | Indirect — convert with a K factor, not reliable |
| Lumens (lm) | lumens | Total human-visible light output | None — do not use for plants |
Three PPFD calculation mistakes UK growers make
These are the errors we see most in UK flat-grow communities. Each one leads to either under-lit plants or wasted energy — and each one is one input correction in the calculator above.
Using headline wattage
A light listed as "2000W LED panel" typically pulls 200–300W at the wall. Using the inflated figure triples your calculated PPFD and leaves plants severely under-lit. Always use true power draw — check the driver label or measure with a plug-in watt-meter.
Ignoring the inverse-square law
Double the distance from canopy to light and PPFD drops to a quarter. A light delivering 400 µmol/m²/s at 30 cm delivers just 100 at 60 cm. Measure — or calculate — at actual canopy height, and lower the light when plants need more light rather than increasing wattage.
One PPFD for all plants
A Monstera thrives at 150 µmol/m²/s. Give it the 500 µmol/m²/s your microgreens need and you'll see bleached, stressed leaves — photoinhibition. Conversely, basil starved of light at 80 µmol/m²/s stays thin and flavourless. Use the plant presets above to check the match.
How to convert lux to PPFD (with a quick converter)
A lux meter is the tool most UK home growers already own. It measures brightness in terms of human perception, but with the right conversion factor (K) you can estimate PPFD from it. The conversion is light-source-specific because each spectrum maps differently to the plant-relevant PAR waveband.
Quick lux → PPFD converter
For a full calculator with DLI and plant suitability, use the Lux → PPFD tab above.
K factors are approximate — light spectrum varies between manufacturers. A dedicated quantum (PAR) sensor removes the guesswork entirely and is the recommended measurement method for any serious UK grow setup. They start at under £50 for a decent handheld model.
People also ask
How much lux is 1000 PPFD?
1,000 µmol/m²/s is roughly 50,000–70,000 lux, depending on your light source. Under a broad-spectrum white LED, 1,000 µmol/m²/s ≈ 70,000 lux (divide lux by 70 to estimate PPFD). Under a red-blue LED, the same PPFD requires only ~50,000 lux, because red and blue photons carry less lumen weighting per photon than green. Always use a light-specific K factor — the universal figure is inaccurate enough to send you seriously off course.
Is PPFD the same as lumens?
No. PPFD measures photons in the 400–700 nm waveband that drive photosynthesis, in µmol/m²/s. Lumens measure brightness as perceived by the human eye — they weight green light (500–600 nm) very heavily, even though plants barely use that portion for photosynthesis. A light with a high lumen output can deliver mediocre PPFD for plants. Red-blue LEDs look dim to our eyes but can deliver excellent PPFD. Never use lumens to compare grow lights.
How to figure PPFD?
The formula: (true watts × efficacy in µmol/J) ÷ grow area in m². A 100 W LED at 2.5 µmol/J over 0.5 m²: (100 × 2.5) ÷ 0.5 = 500 µmol/m²/s. You can also estimate from a lux reading (multiply lux by the K factor for your light type — about 0.0143 for a white LED), or measure directly with a quantum PAR sensor for the most accurate result. The calculator above handles both methods.
How do I convert lux to PPFD?
Multiply your lux reading by the K factor for your light source:
- White / full-spectrum LED: lux × 0.0143 (or lux ÷ 70)
- Red-blue (blurple) LED: lux × 0.0200 (or lux ÷ 50)
- HPS: lux × 0.0122 (or lux ÷ 82)
- Fluorescent T5/T8: lux × 0.0135 (or lux ÷ 74)
- Natural daylight: lux × 0.0185 (or lux ÷ 54)
Example: 20,000 lux under a white LED × 0.0143 ≈ 286 µmol/m²/s. These K factors are approximate — a quantum PAR sensor measures plant-relevant photons directly. K factor data from the University of Maryland Extension and peer-reviewed photobiology literature.
Know your PPFD. Grow with confidence.
The plants that fail in UK flats aren't failing because of bad genetics — they're failing because the light figures were guessed, not calculated. Run the numbers above, then match your setup to the plant's actual target.