- Turn a published DLI range for greens into a PPFD for your photoperiod.
- Read PPF, PPFD, wattage and efficacy as four different numbers.
- Write a spec sheet for coverage, height, spectrum, control and energy, then verify with a meter.
Daily light calculator
DLI = PPFD × hours × 0.0036.
DLI (mol/m²/day): 14.4
Assumes constant PPFD over the stated hours. It does not integrate changing sunlight or prescribe a crop target.
01 / Start from a daily light integral
Purdue HO-238-W defines the daily light integral (DLI) as the photosynthetically active radiation received each day, intensity in µmol·m⁻²·s⁻¹ times duration, in mol·m⁻²·d⁻¹. A grow light is bought against it, so the first line of the specification is a crop-specific range.
Virginia Tech SPES-720’s table of suggested average DLI values lists lettuce at 12–17 mol·m⁻²·d⁻¹, noting that the values are a reference and may need adjustment. Virginia Tech SPES-462 calls about 17 mol/m²/day the optimum for lettuces and warns that excessive light increases tip burn. Cornell’s CEA Hydroponic Lettuce Handbook targets 17 mol/m²/d for the pond stage, matched with downward airflow to prevent tip burn; without it Cornell was not able to go over 12, a result it limits to one cultivar (Ostinata) no longer available. Write 12–17 and choose the end by whether you can move air across the canopy.
Reading: Measuring Daily Light Integral in a Greenhouse (HO-238-W) ↗ · Calculating and Using Daily Light Integral (DLI): An Introductory Guide (SPES-720NP) ↗ · Hydroponic Production of Edible Crops: Management Basics (SPES-462NP) ↗ · Cornell Controlled Environment Agriculture Hydroponic Lettuce Handbook (2013) ↗
02 / PPF, PPFD and watts are three different numbers
Oklahoma State’s LED Grow Lights for Plant Production fact sheet separates the photon figures: PPF is the total light a fixture produces each second; PPFD is the amount reaching a given surface. Spec sheets quote PPF in µmol/s and PPFD in µmol·m⁻²·s⁻¹. University of Minnesota Extension’s small-scale hydroponics page gives the smallest example, a 9-watt LED with a PPF of 16 micromoles per second over one or two buckets; its indoor-lighting page adds that watts measure the energy needed to produce light, not its intensity, and lumens measure brightness to the human eye.
DLI comes from PPFD and time. HO-238-W multiplies a constant PAR reading by 0.0864, the seconds in a day divided by 1,000,000; for a lamp on a timer that is PPFD × hours × 0.0036, which SPES-720 writes as PPFD × 3,600 × operating hours ÷ 1,000,000. Run it backwards: target DLI ÷ (hours × 0.0036) is the average PPFD the fixture must deliver across the whole bed.
Reading: LED Grow Lights for Plant Production (fact sheet; no publication number shown on the page) ↗ · Small-scale hydroponics ↗ · Lighting for indoor plants and starting seeds ↗ · Measuring Daily Light Integral in a Greenhouse (HO-238-W) ↗ · Calculating and Using Daily Light Integral (DLI): An Introductory Guide (SPES-720NP) ↗
03 / Efficacy is the comparable number
When a manufacturer publishes it, efficacy lets two fixtures be compared. Kusuma, Pattison and Bugbee’s 2020 review in Horticulture Research defines it as micromoles of photon output per second per watt of input power, which simplifies to µmol per joule, and gives the scale: a 1,000 W double-ended high-pressure sodium fixture at 1.72 µmol/J, the most efficient LED fixtures it tabulated at 3.0 µmol/J for blue plus red and 2.78 for white plus red, and calculated limits of 4.1 and 3.4 µmol/J.
The review notes that different colors of LED have different efficacies, that efficacy falls about 10 percent as LED temperature rises from 25 to 85 °C, and that timing, angular delivery, spectrum and intensity also determine effectiveness. Ask for PPF and input watts measured on the complete fixture, driver included, and use efficacy as the tie-breaker between fixtures that both deliver your PPFD over your area.
04 / Coverage, mounting height and spectrum
PPF is spread over whatever the fixture lights, so PPFD at a point depends on height, beam angle and spill past the bed. PPF ÷ bed area is an upper bound, not a reading. Ask for a PPFD map at a stated height and compare its average and dimmest point with the PPFD you calculated.
UMN’s indoor-lighting page suggests 6–12 inches between the light and hydroponic lettuce and herbs, 4–6 inches for seedlings; Oklahoma State notes that LED heat leaves through a heat sink rather than the emitting surface, allowing close proximity to plants. Buy an adjustable hanger. Spectrum, in plain terms: UMN’s small-scale hydroponics page says a balance of blue and red light is best for plant growth, that white or full-spectrum bulbs produce that balance, and that leafy greens can get away with just blue. No spectrum claim is made here; buy a spectrum the maker specifies and you can inspect leaves under.
Reading: Lighting for indoor plants and starting seeds ↗ · LED Grow Lights for Plant Production (fact sheet; no publication number shown on the page) ↗ · Small-scale hydroponics ↗
05 / Dimming, timers, heat and the energy line
UMN tells indoor growers to use a timer and lists 12–14 hours per day for hydroponic lettuce and herbs, 16–18 for seedlings. A fixture that reaches your PPFD at full power can be dimmed once raised, or run fewer hours, and land on the same DLI. Specify dimming and a controller set in hours, and record both with every meter reading.
UMN lists high-pressure sodium as releasing a lot of heat and LEDs as not producing too much; the room still receives every watt, so input watts are also the heater rating above your reservoir. Energy is arithmetic: kWh = watts × hours × days ÷ 1,000 at your tariff, as the operating-costs chapter sets out.
Reading: Lighting for indoor plants and starting seeds ↗ · Small-scale hydroponics ↗
06 / Verify with a meter, then write the sheet
HO-238-W measures DLI with a quantum sensor and data logger and warns that a single measurement in time does not represent the light a plant received in a day. Under a timed fixture, a grid of readings at canopy height, averaged and multiplied by hours × 0.0036, is a fair estimate. SPES-720 notes that phone apps use the phone’s internal photodiode and are typically the most cost-effective option; as site guidance, not the source’s, check an app against a quantum meter under your own fixture and use it to compare positions, not for the absolute figure.
Fill the sheet from your bed and the sources named, then ask each supplier for the same items.
- Write the DLI range with its source and pick the end you can support with airflow.
- Fix the photoperiod and compute the required average PPFD: DLI ÷ (hours × 0.0036).
- Measure the bed and the height range the hanger allows.
- Ask each supplier for PPF, input watts, efficacy and a PPFD map at a stated height; set aside sheets that quote only watts or lumens.
- Check that the map’s dimmest point meets the required PPFD with dimming headroom, then add controller, heat and kWh.
- Install, take a grid of quantum-meter readings at canopy height, compute the DLI and record it with the settings.
| Criterion | What to write down | Where the figure comes from |
|---|---|---|
| DLI target | Range in mol·m⁻²·d⁻¹; canopy airflow | SPES-720 12–17; SPES-462 about 17; Cornell 17 with airflow, 12 without |
| Photoperiod and control | Hours on the timer; dimming range | UMN indoor lighting: 12–14 h; the calculator |
| Required PPFD | DLI ÷ (hours × 0.0036), bed average | HO-238-W; SPES-720; the calculator |
| PPF and efficacy | µmol/s and µmol/J, complete fixture | Manufacturer’s sheet; Kusuma et al. scale |
| PPFD map | Average and dimmest point at a stated height | Manufacturer’s map, then your meter grid |
| Height and spectrum | Adjustable range; what the maker specifies | UMN 6–12 in; OSU heat-sink note; UMN blue–red balance |
| Heat and energy | Input watts; kWh = W × h × days ÷ 1,000 | Operating-costs chapter |
| Verification | Grid readings, hours, dimmer setting, date | HO-238-W single-reading warning; SPES-720 on apps |
FIELD QUESTIONA 60 × 60 cm lettuce tray, lights on 14 hours a day, target 12–17 mol·m⁻²·d⁻¹. What average PPFD is required, what PPF would that need if every photon landed on the tray, and how does UMN’s 9 W, 16 µmol/s bulb compare?
14 × 0.0036 = 0.0504. Required PPFD: 12 ÷ 0.0504 ≈ 238 µmol·m⁻²·s⁻¹, 17 ÷ 0.0504 ≈ 337. The tray is 0.36 m², so the idealised PPF is 238 × 0.36 ≈ 86 µmol/s and 337 × 0.36 ≈ 121 µmol/s; a real fixture spills light, so buy from the PPFD map. UMN’s bulb over the whole tray could not exceed 16 ÷ 0.36 ≈ 44 µmol·m⁻²·s⁻¹, or 2.2 mol·m⁻²·d⁻¹; over a single bucket lid of roughly 0.07 m² (a 30 cm circle) it could reach about 16 ÷ 0.07 ≈ 230 µmol·m⁻²·s⁻¹, which fits UMN’s pairing of one bulb with one or two buckets rather than a tray; the lid area is illustrative geometry, not a UMN figure.
Reading: Measuring Daily Light Integral in a Greenhouse (HO-238-W) ↗ · Calculating and Using Daily Light Integral (DLI): An Introductory Guide (SPES-720NP) ↗ · Hydroponic Production of Edible Crops: Management Basics (SPES-462NP) ↗ · Cornell Controlled Environment Agriculture Hydroponic Lettuce Handbook (2013) ↗ · Lighting for indoor plants and starting seeds ↗ · LED Grow Lights for Plant Production (fact sheet; no publication number shown on the page) ↗ · From physics to fixtures to food: current and potential LED efficacy (Horticulture Research 7:56, 2020) ↗ · Small-scale hydroponics ↗
Parts & buying criteria
Build your own parts & cost worksheet →
Showing aerated deep water culture. Quantities describe the teaching model. Specify real working volume, support, fittings and instruments for your installation.
01Nutrient reservoir1 · illustrated quantity+
Stores the measured nutrient solution beneath or beside the growing area.
Inspect: Compare the surface with the diffuser and the lower roots; freeboard is deliberate. There is no pump intake in this single-vessel example.
Maintain: Record level before refilling, keep light out and verify temperature, EC and pH.
Opaque, cleanable, intended-use container with volume marks and service access.
Check size, materials and operating conditions with your chosen supplier before ordering.02Lid & root support1 · illustrated quantity+
Holds the plant above the chamber or reservoir without pinching its crown.
Inspect: Separate the cover and inspect the basket or collar opening.
Maintain: Check fit as plants grow and keep unused openings covered.
Rigid compatible support with removable, correctly fitted baskets or collars.
Check size, materials and operating conditions with your chosen supplier before ordering.03Air pump & diffuser1 · illustrated quantity+
Delivers air below the water surface in this aerated DWC example.
Inspect: Trace the thin line from the dry pump to the diffuser. Bubbles are illustrative.
Maintain: Check actual delivery and dissolved oxygen; preserve backup air and prevent back-siphoning.
Rated output at the installed depth and diffuser resistance; accessible replaceable parts. Power it through ground-fault protection (GFCI/RCD) and keep the pump and its plug above the water line.
Check size, materials and operating conditions with your chosen supplier before ordering.Evidence beside the lesson.
Source checks: September 2026. The geometry, inspection exercises and worksheets are original teaching material. The named organizations have not endorsed or reviewed this site.
Published recipe rates and planning ranges retain their source context. Calculator equations are accounting tools; they do not predict uptake, yield, toxicity or safe stocking. Model dimensions, water speeds, roots and fish counts are illustrative. Verify species, crop, source water and product labels for a real system.
- Purdue University ExtensionMeasuring Daily Light Integral in a Greenhouse (HO-238-W) ↗
DLI, PPFD and light measurement.
- Virginia Cooperative Extension, Virginia TechCalculating and Using Daily Light Integral (DLI): An Introductory Guide (SPES-720NP) ↗
Table of suggested average DLI values for example crops (lettuce 12–17, spinach 14–20, basil 15–25 mol·m⁻²·d⁻¹) with the note that they are a reference and may need adjustment; the electric-light formula DLI = (PPFD × 3,600 × operating hours) ÷ 1,000,000 and its worked example of 200 µmol·m⁻²·s⁻¹ for 16 hours giving 11.5 mol·m⁻²·d⁻¹; quantum sensors measuring PPFD and recording over time when paired with a computer; phone apps using the internal photodiode as typically the most cost-effective option; and the statement that eyes are not reliable light-measuring tools.
- Virginia Cooperative Extension, Virginia TechHydroponic Production of Edible Crops: Management Basics (SPES-462NP) ↗
The statement that the optimum DLI for lettuces is about 17 mol/m²/day and the warning that excessive light levels during production increase the incidence of leaf tip burn and other growth disorders.
- Cornell University CEA Program (M. Brechner and A. J. Both)Cornell Controlled Environment Agriculture Hydroponic Lettuce Handbook (2013) ↗
Pond-stage lighting: supplemental 100–200 µmol/m²/s at plant level for a total of 17 mol/m²/d of natural plus supplemental light; the 17 mol/m²/d target having to be matched with downward airflow to prevent tip burn, and the statement that without that airflow the authors were not able to go over 12 mol/m²/d, with the caveat that the data are for cultivar Ostinata only.
- Oklahoma State University ExtensionLED Grow Lights for Plant Production (fact sheet; no publication number shown on the page) ↗
Definitions of PPF as the total light produced each second by a source and PPFD as the amount reaching a given surface; DLI as moles per square metre per day; quantum sensors and spectroradiometers as the measuring instruments; and LED heat leaving through a heat sink rather than the emitting surface, allowing close proximity between plants and LEDs.
- University of Minnesota ExtensionSmall-scale hydroponics ↗
Root support, transplanting, light, aeration and cleaning.
- University of Minnesota ExtensionLighting for indoor plants and starting seeds ↗
Use of a timer; 12–14 hours per day for hydroponic lettuce and herbs and 16–18 for seedlings; 6–12 inches between light and hydroponic lettuce and herbs and 4–6 inches for seedlings; watts measuring the energy needed to produce light rather than intensity and lumens measuring brightness to the human eye.
- Kusuma, Pattison and Bugbee · Utah State University and Solid State Lighting Services · open access via PubMed CentralFrom physics to fixtures to food: current and potential LED efficacy (Horticulture Research 7:56, 2020) ↗
Efficacy defined as micromoles of photon output per second per watt of input power, simplifying to µmol per joule; the 1,000 W double-ended HPS figure of 1.72 µmol/J; tabulated 2020 fixture efficacies of 3.0 (blue plus red) and 2.78 (white plus red) µmol/J; calculated limits of 4.1 and 3.4 µmol/J; different LED colors having different efficacies; efficacy falling about 10 percent from 25 to 85 °C; and the statement that timing, angular delivery, spectrum and intensity also determine effectiveness.
The next useful connections.
Leafy greens and herbs: plan a repeatable crop
Choose a first crop, manage spacing and harvest timing, and avoid forcing every herb into lettuce conditions.
6 min + guided practice →Crops & environmentLight, temperature and humidity
Measure the growing environment at the canopy and root zone, then adjust one limiting condition at a time.
Interactive tool · 6 min + guided practice →Build, operate & troubleshootOperating costs and sensible scaling
Build a complete cost ledger, measure energy and labor, and expand from usable production evidence.
Interactive tool · 6 min + guided practice →ComparisonsKratky vs DWC for lettuce: pick by power, season and scale
Both methods hang lettuce roots in stored nutrient solution; they differ in where oxygen comes from, how much water each plant needs on day one and what fails first.
7 min + guided practice →ComparisonsReady-made kits vs DIY hydroponics: sites, light, pods and repairs
A boxed countertop garden and a drilled bucket meet the same needs; they differ in who chooses the light, the consumables, the meter and the spares.
8 min + guided practice →