Calculator
Formula used
Footcandles from lux: fc = lux ÷ 10.76391
Lux from footcandles: lux = fc × 10.76391
Lumens (optional): since lux = lumens / m², then lumens = lux × area(m²).
Note: Lux and footcandles are illuminance units. Plant response depends on spectrum, distance, and photoperiod, so treat targets as practical starting points.
How to use this calculator
- Measure lux at canopy height where leaves receive light.
- Choose the direction that matches your meter reading.
- Optionally enter the lit area to estimate total lumens.
- Pick a plant profile to see a target lux range.
- Press Calculate, then export CSV or PDF if needed.
Example data table
| Scenario | Lux | Footcandles | Typical gardening use |
|---|---|---|---|
| Low-light corner | 2,000 | 186 | Shade-tolerant houseplants |
| Seedling shelf | 8,000 | 743 | Propagation and young starts |
| Leafy greens zone | 20,000 | 1,858 | Fast, compact vegetative growth |
| Fruiting bench | 35,000 | 3,252 | Flowering and fruit development |
Lux and Footcandle Meaning for Grow Areas
Lux and footcandles describe illuminance on the crop surface. Lux is lumens per square meter, while footcandles are lumens per square foot. Because many horticulture charts still use footcandles, fast conversion prevents under‑lighting. In practice, measure at canopy height, average several points, and keep the sensor level. Log time, sky, and sensor orientation. Stable readings improve decisions about hanging height and lamp spacing.
Crop Lighting Targets and Practical Ranges
Different crops tolerate different light intensity. Leafy greens often perform well around 15,000–25,000 lux, while herbs commonly need 10,000–20,000 lux for compact growth. Fruiting plants may push 25,000–50,000 lux depending on cultivar and photoperiod. Use the plant profile to compare your reading with a target band, then adjust fixtures gradually to avoid stress. Aim for even coverage, not peak hotspots.
Measurement Quality and Common Meter Issues
Light meters and phone sensors can drift, especially under LED spectra. To reduce error, warm up fixtures, avoid shadows from your body, and take readings when fans are steady. If you switch between lux and footcandle modes, confirm the meter’s calibration factor. This calculator uses 10.76391 lux per footcandle, matching standard area conversion between square meters and square feet. Map readings along rows for uniformity.
Using Area to Estimate Total Lumens Needed
When you enter area, the calculator estimates lumens using lumens = lux × area(m²). This is useful for rough sizing: if a bench is 2.0 m² at 20,000 lux, you need about 40,000 lumens delivered to that surface. Real systems lose light to distance, reflectance, and optics, so treat lumens as a planning baseline, not a guarantee. Reflective surfaces can improve delivered lumens.
Action Steps After You Convert and Compare
After calculating, focus on the target status and the lux difference. If you are below target, reduce mounting height, increase fixture count, or extend daily runtime. If you are above target, raise fixtures or shorten runtime to limit heat and photo‑stress. Re‑measure weekly as plants grow, because canopy height changes intensity and uniformity across the growing zone. Export results to track seasonal lighting changes.
FAQs
What is the exact conversion between lux and footcandles?
One footcandle equals 10.76391 lux. To convert lux to footcandles, divide by 10.76391. To convert footcandles to lux, multiply by 10.76391.
Where should I take light readings for plants?
Take readings at canopy height, directly above the leaves, and sample multiple points across the growing area. Average the values to judge uniformity and avoid being misled by a single bright spot.
Why does my lux reading change when plants grow taller?
As the canopy moves closer to the light, intensity increases. Leaves can also shade lower layers. Re-check lux after pruning, training, or raising trays so you keep the target range at the active leaf surface.
Does this calculator tell me PPFD or plant-usable light?
No. Lux and footcandles measure visible-weighted illuminance, not photosynthetic photon flux. Use this tool for practical brightness comparisons and legacy charts, then confirm performance by observing plant compactness, color, and internode length.
How do I use area and lumens in a greenhouse plan?
Enter area to estimate lumens using lumens = lux × m². Compare that lumen need with fixture output and account for losses from mounting height and reflectors. It is best for sizing and quick comparisons.
What should I adjust first if I am below target?
Start by improving uniformity: reposition fixtures, reduce mounting height, and eliminate shadows. If readings stay low, add fixtures or increase runtime. Make changes gradually and measure again to confirm improvement.