A planting site’s brightness matters less than how much total light it delivers over a day. This calculator converts your sun hours and an estimated photon flux into a Daily Light Integral — the figure that actually drives plant growth — and tells you what will thrive there.
Why DLI matters more than “full sun” or “part shade”
Gardening labels like “full sun” (six or more hours of direct sunlight) are useful shorthand but imprecise. Two sites might both qualify as “full sun” but deliver very different amounts of usable light: a high-latitude garden in June getting six hours of sun at a low angle delivers far fewer photons than a south-facing bed in the same conditions because the sun angle affects intensity. DLI captures both the duration and the intensity, collapsing them into the single figure that plants actually respond to.
Plants do not respond to how bright the light looks — they respond to how many photons in the photosynthetically active radiation range (400–700 nm wavelength) arrive each day. DLI measures exactly that, in moles of photons per square metre per day.
The conversion in detail
DLI is PPFD (instantaneous photon flux) integrated over the photoperiod. Because a mole is one million micromoles, and there are 3,600 seconds in an hour:
DLI (mol/m²/day) = PPFD (µmol/m²/s) × hours × 3600 / 1,000,000For example, 6 hours of afternoon sun at an effective average PPFD of 1,500 micromoles:
1,500 × 6 × 3,600 / 1,000,000 = 32.4 mol/m²/dayThat 32.4 mol/m²/day falls comfortably in the full-sun zone and suits fruiting vegetables, cut flowers, and most herbs.
Estimating PPFD for your site
PPFD is the trickiest input because outdoor light is not constant — it varies with the angle of the sun through the day, cloud cover, and obstructions. The figure to enter is an effective average for the hours of direct sun, not the peak.
| Condition | Approximate PPFD range |
|---|---|
| Full, unobstructed summer midday | ~1,800–2,000 µmol/m²/s |
| Open site, morning or afternoon sun | ~1,000–1,500 µmol/m²/s |
| Bright overcast sky | ~400–800 µmol/m²/s |
| Dappled shade under a tree | ~200–500 µmol/m²/s |
| Dense shade, north-facing wall | ~50–200 µmol/m²/s |
For an open garden bed receiving six hours of direct sun that is mostly mid-morning to mid-afternoon (not peak noon sun), an effective average of around 1,200–1,400 µmol/m²/s is reasonable. A south-facing patio that catches the full midday arc can average 1,500–1,800.
Plant DLI thresholds at a glance
These are broad reference ranges — individual species vary significantly:
| Category | Typical DLI needed | Examples |
|---|---|---|
| Full-sun crops | 20–40 mol/m²/day | Tomatoes, peppers, cucumbers, roses |
| Leafy greens and herbs | 10–20 mol/m²/day | Lettuce, basil, spinach, parsley |
| Part-shade perennials | 6–12 mol/m²/day | Hostas, ferns, astilbe |
| True shade plants | 2–6 mol/m²/day | Mosses, some ferns, woodland groundcover |
| Below ~2 mol/m²/day | Survival only | Most plants decline rather than thrive |
Practical limitations
This calculator estimates DLI from inputs you provide — it does not measure it. A proper quantum meter or PAR logger mounted at your site and logging through the full day gives a real DLI figure. The estimate here is well suited for quickly judging whether a proposed planting site is viable before you dig or plant, but it should not substitute for measurement in a controlled or commercial growing context.