Supplemental Lighting Planner: How Much Light Is Your Space Missing?
Most grow light buying guides start from a fixture and work backwards to a room. That is the wrong end of the problem for anyone who already has plants in a space that is not a grow tent.
An office with planters along a corridor. A living room with a fiddle leaf fig that stopped growing last November. A restaurant window display. A villa atrium with a mature specimen plant in a heavy pot. A spare room that has become a grow room.
In all of these the lamp is not the whole question. The question is how much light the space already provides, how much the plant needs, and therefore how much is actually missing. That number is often much smaller than a fixture spec sheet would have you buy, and occasionally it is zero.
The planner
Sets the ambient estimate, hours, and mounting height below. Everything stays editable.
Area the plants occupy m² ft² The planted footprint, not the room floor. Type in either unit.
lux Measured with a meter in the evening, or estimated from the space type above. Bright office is 300–500; a bright living room 1,000–2,000; a metre inside a sunny window 8,000–15,000.
W Rated wall power, not light output.
Modern quantum-BOARD fixtures: 2.4–3.2. Older or budget panels: 1.2–1.8.
cm Fixtures are rated at 30 cm. Mounted higher, less reaches the canopy.
- Ambient contribution
- —
- Target for this plant
- —
- Light output needed
- —
- Fixtures at your wattage
- —
- Total wall power
- —
Send these numbers to the factory Your crop, room, light levels and the calculated fixture count arrive in the message, so the first reply can be specific.
These are reference values, not a specification. Real light also depends on the fixture's spectrum, room reflectance, and how densely the plants are packed. Everything runs in your browser — nothing is sent anywhere.
What it is actually doing
Four steps, all of which you can check by hand.
- Ambient light to photons. A lux reading is converted with the standard daylight coefficient,
0.0185. A 500 lux office becomes about 9 µmol/m²/s. - That over a day. Ambient DLI is the intensity above multiplied by hours of light and by
0.0036. For 500 lux over 12 hours, about 0.4 mol/m²/day. - Compare against the plant. Leafy greens want 12–17. Lettuce under typical office lighting is getting roughly 3% of what it needs. That number is the point of the exercise.
- Size the gap. The deficit is converted to the light output required at the canopy, then divided by fixture efficacy to give you a wall-power figure and a fixture count.
Why lux is the wrong number for a grow light
The conversion above is valid for daylight and other broadband sources. It is not valid for plant lighting, and the tool says so at the top for a reason.
A lux meter is calibrated to the visible spectrum weighted toward human vision. Grow light fixtures push energy into narrow red and near-infrared bands. Two fixtures of identical photon output can read completely different values on the same meter, and a working fixture can read as almost dark. People routinely measure a functioning grow light at under 300 lux and conclude the lamp is broken.
A plant does not respond to lux either. It responds to photons in the 400–700 nm band, weighted by absorption. Lux is a proxy for daylight, and a proxy stops being useful when the source changes that much.
So this tool uses lux only for what lux is good at — characterising the light already in your room. Everything after that is in photon terms, where fixtures are actually specified.
The reference values
These are ranges, not point values, and they are the weakest part of any calculator like this one.
| Group | Target DLI | Typical photoperiod | Intensity ceiling |
|---|---|---|---|
| Leafy greens (lettuce, arugula) | 12–17 | 14–16 h | ~250 µmol/m²/s |
| Fruiting vegetables (tomato, pepper) | 20–30 | 14–16 h | ~600 µmol/m²/s |
| Root crops (radish, carrot) | 10–16 | 14–16 h | ~400 µmol/m²/s |
| Dragonfruit / pitaya | 15–20 | 13–14 h to flower | ~500 µmol/m²/s |
| Other tropical fruit | 18–26 | 14–16 h | ~500 µmol/m²/s |
| Flowering ornamentals (jasmine, gardenia) | 16–24 | 12–16 h | ~500 µmol/m²/s |
| Medium-light foliage (monstera, ficus) | 8–14 | 14–16 h | ~300 µmol/m²/s |
| Bright-light foliage | 14–20 | 14–16 h | ~400 µmol/m²/s |
| Succulents and cacti | 12–16 | 10–14 h | ~400 µmol/m²/s |
| Basil | 14–20 | 16–18 h | ~400 µmol/m²/s |
| Mint | 14–20 | 16–18 h | ~400 µmol/m²/s |
| Parsley, coriander | 12–18 | 14–16 h | ~400 µmol/m²/s |
| Seedlings and cuttings | 4–8 | 16–18 h | ~200 µmol/m²/s |
| Microgreens | 8–12 | 14–16 h | ~250 µmol/m²/s |
Where these are solid. The intensity ceilings are the most defensible numbers here. For lettuce, work by Sago and colleagues published in HortScience tested butterhead at 150, 200, 250 and 300 µmol/m²·s⁻¹ and found that while dry weight rose with intensity, tipburn rose with it too — which is why commercial guidance generally keeps lettuce below 300, and often below 250. The succulent photoperiod and the dragonfruit flowering threshold are equally well established.
Where they are weaker. Published indoor DLI figures for flowering ornamentals disagree badly: Iowa State Extension groups flowering houseplants at 12–16 mol, while other sources put “high light” plants at 20–40. Nobody has published a rigorous indoor figure for gardenia specifically. The planner uses 16–24 and would not be offended if you took the lower end first and adjusted from what the plant does.
We would rather show you a range and tell you where the uncertainty is than print a number that looks precise and is not.
When more light is the wrong answer
A light target is a floor and a ceiling, and most advice on this topic only mentions the floor.
Lettuce is the clean example. Push intensity past roughly 250–300 µmol/m²/s and dry weight still goes up — so the plant still looks productive — while tipburn goes up with it. Tipburn is a physiological disorder, not a cosmetic one: the outer leaf margins die back, the head is no longer saleable, and in a home grow it just means the salad is not worth eating.
Fiddle leaf fig and gardenia do the same thing in summer heat, with brown patches on the leaves facing the lamp.
So the planner will sometimes tell you that the configuration it just computed is brighter than your plant can comfortably take, and recommend more hours or more distance instead. That is not a hedge. Extending the photoperiod delivers the same daily total for less instantaneous stress, and often for a cheaper fixture.
Succulents invert the rule. They want the opposite of everything else on this page: high intensity over a short day. A succulent run for 18 hours at low intensity stretches and loses its compact shape. Ten to fourteen hours at higher intensity is the correct configuration, and it is why the tool reports a preferred range for hours instead of just accepting whatever you type.
When hours are a switch, not a dial
Most plants respond to a shortfall in light gradually. Dragonfruit does not.
Indoor pitaya needs roughly 13–14 hours of light before it will switch into flowering at all. Run it at 8, 9 or 10 hours and no amount of extra intensity produces a flower — the photoperiod is a threshold, not a dial, and the plant is waiting for the day to be long enough. This is the single most common reason indoor dragonfruit never flowers, and it has nothing to do with the lamp.
If you are growing dragonfruit, set your hours to at least 13 before you spend anything on output. The planner flags this separately for that reason. Our dragonfruit guide covers the rest of the setup.
Offices, villas and retail: the same question in different clothes
The calculation is identical across all of these. What changes is the constraint, and each one produces a different answer.
Offices. Typical open-plan illumination is 300–500 lux, which for almost any plant is a rounding error — the planner puts lettuce at about 3% of its requirement. The practical obstacle is not light, it is that the lights are on for 8–10 hours while plants want 14+, and nobody wants a lamp blazing over a meeting table. Low-profile, shielded fixtures on a separate timer circuit are usually the answer.
Villas and double-height spaces. Here the constraint is mounting height. A 3.5-metre ceiling changes everything: light falls off with distance, the required rated output climbs steeply, and heat rises. Our advice is to bring the light down rather than buy a bigger one — a fixture on a drop rod or over a planting bed, not a high-bay aimed at a canopy 2.5 metres below. The planner’s height adjustment will show you how much that matters; it uses inverse-square, which is conservative for a panel but shows the right direction.
Retail and restaurant displays. The challenge is consistency. A display lit for four hours a day on a schedule will look very different from one lit for twelve, and plants next to a window are never the same as plants two metres in. Keeping plants in one zone with its own lighting, rather than scattered along a frontage, is the cheapest improvement available.
Where this tool stops
This planner answers one question: how much is missing from what you have. It does not size a fixture, and it is not a substitute for a photometric measurement.
Once you know the output you need at the canopy — the planner gives you that number in µmol/s — take it into the PPFD and DLI calculator, which works from a fixture’s rated output across your area and hours. That calculator assumes you are specifying a fixture for a growing area, which is the correct model for a new installation and the wrong one for a room that already has light.
If you are building a commercial installation, mapping the whole growing area with a quantum sensor beats both of these tools. A grid map will show you the dark corners that an average conceals.
Related reading
- PPFD and DLI calculator — sizing a fixture for a growing area, in photons
- Desk and small-space grow light setup — mounting, height and timers on a shelf
- How many watts for desktop plants — the wattage shortcut when you want it
- Dragonfruit indoors — photoperiod-triggered flowering
- The complete indoor gazania guide — a flowering ornamental worked through in detail
- Indoor basil guide — a herb crop at length
FAQ
How many lux do office plants need?
Most indoor plants are fine at 500–5,000 lux and do well at 5,000–10,000. Standard office lighting at 300–500 lux is below what almost any plant would choose for itself, which is why office planters need supplemental light. If a plant merely has to survive, 500 lux is enough. If you want it to actually grow, budget for light.
Can I use a lux meter on my grow light?
Not meaningfully. Lux meters are calibrated for daylight, grow fixtures are narrow-spectrum, and the reading will be far lower than the actual photon output — sometimes low enough to suggest a broken lamp. Use a lux meter for ambient light and a quantum or PAR meter for the lamp.
Do I need grow lights if I have a bright window?
Sometimes not. Measure or estimate what the window delivers at leaf height rather than at the glass — a metre inside a sunny window is often 8,000–15,000 lux, which is genuinely a lot, and a plant sitting there may need nothing. The catch is seasonality: a window that is excellent in June is poor in December.
Why does my plant need more hours rather than a brighter lamp?
Because every plant has an intensity it can absorb without damage, and raising intensity past that point causes stress while the daily total barely moves. Lettuce is the clearest case, where high intensity increases tipburn. Extending the photoperiod reaches the same total with less instantaneous stress and often a cheaper fixture.
How far above my plants should a supplemental fixture hang?
12–18 inches for a 20–40W fixture, and closer for smaller ones. Mounting higher always reduces what reaches the canopy, which is why the planner adjusts for height — and why bringing a light down is usually a better move than buying a larger one.
What if the calculator says I need no supplemental light?
Then buy nothing. That is a real answer and an honest one. Check that your lux figure is measured at leaf height rather than at the window, and consider whether winter will be worse than the reading you used, because a space that only just meets the target in summer may not in December.