Grow Light Calculator

The Grow Light Calculator estimates PPFD and DLI for 5 growth stages and 5 light types (LED, HPS, fluorescent, MH, CFL). Shows sufficiency indicator, energy cost, heat output, and recommended fixtures. SVG PPFD comparison chart — a feature an AI chatbot cannot replicate. Free, no sign-up required.

Setup Presets

Light Setup

Unit System

Grow Area Length

Grow Area Width

Growth Stage

Light Type

Wattage per Fixture

Number of Fixtures

Height (from canopy)

Daily Light Hours

Results

Insufficient — add more light

Area

16 ft² / 1.49 m²

Estimated PPFD

µmol/m²/s

Target PPFD

400

µmol/m²/s

DLI (Daily Light Integral)

2.7

mol/m²/day

Target DLI

mol/m²/day

Total Watts

300

Watts per sq ft

18.8

W/ft²

Daily Energy

4.8

kWh

Monthly Energy

144

kWh

Monthly Cost (est.)

$17.28

USD

Heat Output

358

BTU/h

Recommended Total Watts

2560

Recommended Fixtures

PPFD: Estimated vs Target

Light Coverage Diagram

Growth Stage Requirements

Stage	PPFD	DLI	Hours/Day
Seedling	200 µmol/m²/s	12 mol/m²/day	14–18 h
Vegetative	400 µmol/m²/s	20 mol/m²/day	16–18 h
Flowering	600 µmol/m²/s	30 mol/m²/day	12–14 h
Fruiting	700 µmol/m²/s	35 mol/m²/day	12–14 h
Low-Light / Houseplant	100 µmol/m²/s	6 mol/m²/day	10–14 h

Light Type Comparison

Type	Efficacy	Heat %	PPFD/W/sqft
LED	2.7 µmol/J	35%	2.5
T5 Fluorescent	1.3 µmol/J	60%	1.2
HPS (High Pressure Sodium)	1.7 µmol/J	70%	1.7
Metal Halide	1.4 µmol/J	65%	1.4
CFL	0.9 µmol/J	70%	0.8

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What is a Grow Light Calculator?

A grow light calculator determines if your indoor lighting setup provides enough photosynthetically active radiation (PAR) for your plants. It measures this as PPFD (Photosynthetic Photon Flux Density, µmol/m²/s) and DLI (Daily Light Integral, mol/m²/day). Different growth stages require different light levels: seedlings need about 200 PPFD, while flowering plants need 600+ PPFD. The calculator also estimates energy consumption and heat output for ventilation planning.

How to Use This Calculator

Select a preset or enter your grow area dimensions
Choose the growth stage to set the target PPFD and DLI
Select your light type (LED, HPS, fluorescent, etc.)
Enter the wattage per fixture and number of fixtures
Set the daily light hours for your photoperiod
Check if PPFD is sufficient — if not, add more fixtures or wattage

Frequently Asked Questions

How many watts of grow light do I need per square foot?

For LED lights: seedlings need 20–25 W/sq ft, vegetative growth needs 25–30 W/sq ft, and flowering needs 30–40 W/sq ft. HPS lights need roughly 40–50% more wattage than LED for the same PPFD output due to lower efficiency.

What is PPFD and why does it matter?

PPFD (Photosynthetic Photon Flux Density) measures the number of photosynthetically active photons hitting a surface per second per square meter. It is the most accurate way to measure useful light for plants. Target PPFD: seedlings 200, veg 400, flowering 600, fruiting 700 µmol/m²/s.

What is the difference between DLI and PPFD?

PPFD is an instantaneous measurement (µmol/m²/s), while DLI is the total daily dose (mol/m²/day). DLI = PPFD × hours × 3600 ÷ 1,000,000. A plant getting 400 PPFD for 16 hours receives a DLI of 23.0 mol/m²/day. DLI better represents total light energy over a photoperiod.

Are LED grow lights more efficient than HPS?

Yes. Modern LED grow lights produce about 2.5–3.0 µmol/J, while HPS produces about 1.7 µmol/J. LEDs also generate 35% heat versus 70% for HPS, reducing cooling costs. However, quality LED fixtures have higher upfront costs than HPS.