- Calculate DLI with explicit constant-light assumptions.
- Map light and log air/root temperatures.
- Relate environmental changes to irrigation and energy.
Daily light calculator
DLI = PPFD × hours × 0.0036.
Assumes constant PPFD over the stated hours. It does not integrate changing sunlight or prescribe a crop target.
01 / Measure photons at the crop
PPFD describes the rate of photosynthetically active photons arriving at a surface, in µmol/m²/s. DLI adds the light over a day, in mol/m²/day. For a constant measured PPFD, DLI = PPFD × hours × 0.0036. At 250 µmol/m²/s for 16 hours, DLI is 14.4 mol/m²/day.
This is a light-accounting example, not a target for every crop. Sunlight varies through the day, so a single noon reading multiplied by daylight hours overestimates or otherwise misrepresents the integral. Use logging measurements or a suitable daily-light instrument.
Reading: Measuring Daily Light Integral in a Greenhouse (HO-238-W) ↗
02 / Map the space
Measure across the canopy at its actual height, including edges and shaded positions. Fixture watts and lumens do not describe the photons arriving at each leaf. As plants grow toward lights, recheck spacing, uniformity and heat.
Keep an air sensor shielded from direct radiant heat and measure root-zone water separately. Record daily high and low values rather than relying only on the comfortable conditions during a morning inspection.
- Draw a simple grid over the occupied growing area and measure PPFD at a consistent height.
- Calculate average and identify low and high zones. Adjust fixture placement or crop spacing, then remeasure.
- Record the actual light schedule; use the calculator only for a constant PPFD assumption.
- Log canopy air temperature, reservoir temperature and humidity through a representative day and night.
- Change one condition and evaluate crop response over a meaningful interval.
03 / Roots and air are different environments
Warm nutrient water holds less dissolved oxygen at saturation and changes the root environment even when the room feels acceptable. Keep reservoirs protected from excessive radiant heat and inspect aeration and circulation during the warmest part of the cycle.
Humidity and airflow affect transpiration, leaf drying and nutrient movement. Air movement should reach the canopy without continuously desiccating the smallest plants. Avoid treating one relative-humidity number as a complete disease or transpiration diagnosis.
Reading: Hydroponic Nutrient Solutions (G6984) ↗ · Small-scale hydroponics ↗
04 / Evaluate controls together
More light may increase crop demand for water and nutrients while adding heat. Ventilation may change temperature, humidity and energy consumption at the same time. Record the controller setting and the measured result; a thermostat setpoint is not proof of actual crop conditions.
Before buying additional fixtures or cooling, identify the limiting period and location. Compare the expected improvement with measured electrical consumption and usable crop output.
FIELD QUESTIONDoes 250 PPFD at one point prove the whole bed receives a 14.4 DLI?
No. The example also assumes 16 hours at constant intensity and that the reading represents the occupied area. Map the canopy and account for changing sunlight or dimming.
Parts & buying criteria
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 pump intake or lower roots; freeboard is deliberate.
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.
Supplier links can be added by the publisher. The criteria stand independently.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.
Supplier links can be added by the publisher. The criteria stand independently.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.
Supplier links can be added by the publisher. The criteria stand independently.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.
- University of Missouri ExtensionHydroponic Nutrient Solutions (G6984) ↗
Solution preparation, elemental nutrition and measurement.
- University of Minnesota ExtensionSmall-scale hydroponics ↗
Root support, transplanting, light, aeration and cleaning.
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