Key Takeaways
- ✅ What it calculates: Daily light hours, dark hours, lights-on/off times, DLI target, actual DLI from PPFD, weekly electricity cost, and monthly cost — for your specific crop and stage.
- 📋 30+ crop profiles: Every crop has researched photoperiod requirements for Seedling, Vegetative, Flowering, and Pre-harvest stages — select yours for a fully tailored schedule.
- ⚡ Electricity cost included: Enter your light wattage and electricity rate to see exactly what your light schedule costs per week and per month — critical for optimising running costs.
- 📊 DLI gauge: See how your actual light dose compares to your crop’s ideal range — the metric commercial growers use to maximise yield, not just hours on a timer.
- 💡 Pro Tip: A consistent schedule beats a perfect one. Even one day of irregular on/off timing can trigger premature flowering in short-day crops like cannabis or strawberries. Always use a digital timer with battery backup.
Last updated: May 2026 by James
💡 Light Schedule Calculator
Select your crop, stage, and fixtures — get your daily schedule, DLI, on/off times, and electricity cost in seconds.
Type to filter the dropdown below.
Stage changes the recommended photoperiod.
When do your lights turn on each day?
Used for electricity calculations in the Cost tab.
From your light’s spec sheet or PAR meter reading.
Width × Length of your grow space in metres.
Distance from lamp to top of plant canopy.
Check your fixture label or spec sheet.
Average US rate ~$0.13–0.16/kWh. UK ~£0.24/kWh.
LED drivers ~8–12% loss. HPS ballasts ~15–20% loss.
Select a crop in the Basic Schedule tab, then calculate to see a full comparison of all four growth stages side-by-side.
Your Light Schedule — —
What is a light schedule and why does it matter for indoor plant growth?
A light schedule — or photoperiod — is the daily pattern of light and darkness your plants receive, expressed as hours on/hours off (for example, 18/6). It controls two critical processes simultaneously: the rate of photosynthesis during the light period, and the hormonal flowering trigger during the dark period. Indoor growers using hydroponic systems or soil have complete control over both.
Plants evolved to use day length as their most reliable seasonal signal. Short-day crops like cannabis and strawberries require a minimum of 12 uninterrupted dark hours to trigger and sustain flowering — a single minute of light during that dark window can delay the harvest cycle by weeks. Day-neutral crops like tomatoes and cucumbers flower regardless of photoperiod, but still respond dramatically to the total light dose (DLI) they receive each day. Getting your DLI and PPFD dialled in alongside your photoperiod is what separates average grows from high-yield results. In our NFT system, switching from a loose timer to a precision digital controller with battery backup increased consistent fruiting on tomatoes by over 20% across a 12-week cycle.
The wrong schedule causes problems that even perfect pH management, EC control, and nutrient mixing cannot fix. A vegetative plant given only 12 hours will remain stunted. A flowering cannabis plant given 14 hours will revert to vegetative growth. This calculator removes the guesswork by giving you researched photoperiod targets for 30+ crops across all four growth stages.
How to use this light schedule calculator
The calculator has four tabs — Basic Schedule, DLI & Intensity, Electricity Cost, and Stage Comparison. Fill in the Basic Schedule tab at minimum; the other tabs add precision to your results.
- Search or select your crop — type in the search box to filter the dropdown. Each of 30+ crops has a researched photoperiod for every growth stage built in.
- Choose your growth stage — Seedling, Vegetative, Flowering, or Pre-harvest. This is the single biggest factor in the recommended schedule. Many crops require a stage change to trigger flowering.
- Select your light fixture type — Modern full-spectrum LEDs often need fewer hours than older blurple LEDs or HPS to deliver the same DLI, because their efficiency (µmol/J) is higher.
- Set your lights-on time — the calculator builds the complete daily on/off schedule around this time. Most growers run lights during daytime hours to simplify temperature management.
- Enter PPFD and wattage (recommended) — go to the DLI tab to enter your PPFD reading from a PAR meter or spec sheet. Go to the Cost tab to enter your wattage and electricity rate. Both unlock additional result cards.
- Read the Stage Comparison tab — after calculating, the Stage Comparison tab auto-populates a table showing all four growth stages for your selected crop side-by-side, so you can plan your full grow cycle in one view.
Ideal light schedules by crop and growth stage
| Crop | Seedling | Vegetative | Flowering / Fruiting | Photoperiod Type | Notes |
|---|---|---|---|---|---|
| Lettuce | 16h | 16–18h | 16h | Day-neutral | Harvest before long days trigger bolting. Check yield estimator at cut. |
| Basil | 16h | 16–18h | 12–14h | Day-neutral | More light = more leaf mass. Reduce to delay bolting. |
| Spinach | 14h | 14–16h | Keep below 14h | Long-day (bolts) | Bolts rapidly above 14h. Keep cool and short for leaf production. |
| Tomato | 16–18h | 18h | 12–16h | Day-neutral | High DLI demand throughout. Monitor EC/TDS at fruiting stage. |
| Pepper / Chili | 16h | 16–18h | 12–14h | Day-neutral | Slight reduction at flowering improves fruit set and quality. |
| Cucumber | 16h | 18h | 14–16h | Day-neutral | Very high light demand. One of the highest DLI crops in indoor growing. |
| Strawberry | 14h | 14–16h | 12h strict | Short-day | Must reduce to exactly 12h to trigger flowering. Critical transition. |
| Cannabis (photoperiod) | 18–20h | 18h | 12h strict | Short-day | Any light during the 12h dark period can revert to vegetative growth. |
| Broccoli | 14h | 14–16h | 12h | Long-day | Requires vernalisation (cold exposure) to form heads — complex indoors. |
| Microgreens | — | 12–16h | — | Day-neutral | Harvested before flowering. T5 fluorescents sufficient — DLI 10–14. |
| Mint | 16h | 16h | 14h | Day-neutral | Pinch flower heads off to keep in continuous leaf production mode. |
| Orchid | 12h | 12–14h | 10–12h | Short-day | Needs 12+ weeks of 10–12h nights to initiate a flower spike. |
DLI targets and electricity cost by crop — the numbers that matter
DLI (Daily Light Integral) is the total photon dose your plants receive per day, measured in mol/m²/day. It combines photoperiod length AND light intensity (PPFD). Use this table alongside the electricity cost calculator tab to plan your running costs at target DLI levels.
| Crop / Category | Ideal DLI (mol/m²/day) | Typical PPFD at 18h (µmol/m²/s) | Estimated Wattage Needed* |
|---|---|---|---|
| Microgreens | 10–14 | 150–215 | 100–200W |
| Lettuce / Leafy greens | 14–20 | 215–308 | 150–300W |
| Herbs (basil, mint, parsley) | 16–22 | 246–338 | 200–350W |
| Spinach / Kale | 14–18 | 215–277 | 150–280W |
| Tomato / Pepper (vegetative) | 22–30 | 338–462 | 300–500W |
| Tomato / Pepper (fruiting) | 28–35 | 431–538 | 400–600W |
| Cucumber | 25–35 | 385–538 | 350–600W |
| Strawberry | 16–22 | 246–338 at 12h | 300–450W at 12h |
| Cannabis (vegetative, 18h) | 35–50 | 539–770 | 500–900W |
| Cannabis (flowering, 12h) | 35–45 | 810–1042 | 700–1200W |
| Orchids | 10–15 | 154–231 | 80–180W |
*Wattage estimates assume a 1m² canopy using modern full-spectrum LED at 2.5 µmol/J efficiency. Actual requirements vary by fixture and grow space size. Use the Electricity Cost tab to calculate your specific setup.
Light schedule problems — causes and fixes
| Problem | Likely Cause | Fix |
|---|---|---|
| Plant not flowering despite 12/12 schedule | Light leak during the dark period — even 1–2 lux during the 12h dark window disrupts the hormonal flowering signal | Inspect every door seam, cable entry point, and equipment LED indicator. Use black-out tape on all indicator lights. A torch in complete darkness reveals leaks instantly. |
| Lettuce bolting (going to seed early) | Photoperiod above 14–16h combined with temperatures over 24°C — accelerates reproductive phase | Reduce light hours to 14h. Lower temperature below 23°C. Harvest outer leaves continuously. Use bolt-resistant varieties for warm setups. |
| Slow growth despite 18+ hours of light | Low DLI — the schedule is long but light intensity (PPFD) is too low to drive sufficient photosynthesis. Hours ≠ dose. | Measure PPFD with a PAR meter. Raise lights closer to canopy (within safe range), increase fixture wattage, or add supplemental lighting. Check DLI calculator to confirm target. |
| Leaf bleaching / tip burn at canopy | PPFD too high — modern LEDs run cool and can be placed close, making light burn easy to miss until damage appears | Raise lights by 5–10cm increments over 3–4 days. Check DLI against crop maximum in the table above. Increase airflow — VPD management via our VPD Calculator reduces heat stress. |
| Leggy, stretched seedlings | DLI below 12 mol/m²/day — seedlings reach toward inadequate light, elongating stems before root capacity is established | Lower lights (not increase hours), upgrade fixture, or reduce ceiling distance. Target 12–16 mol/m²/day at seedling stage. High VPD combined with low light worsens stretching. |
| Uneven fruiting across the canopy | Uneven PPFD distribution — plants at canopy edges receive 30–50% less light than centre plants, creating irregular development | Rotate plants every 3–5 days. Line walls with reflective Mylar (95% reflectivity vs. 80% for white paint). Add edge lighting for canopy coverage above 1.5m². |
| Premature flowering in vegetative crop | Photoperiod accidentally reduced below flowering threshold — common when timer batteries fail or clocks drift | Use a digital timer with battery backup and check weekly. Gradually increase light hours by 1h per day (not sudden jumps) to transition back to vegetative schedule without stress. |
LED vs HPS vs fluorescent — which light type suits your setup?
💡 Full-spectrum LED Most efficient
Modern quantum-board LEDs deliver 2.5–3.0 µmol/J — the highest efficiency of any fixture type. They run cool enough to place 20–30cm from the canopy without heat burn risk, last 50,000+ hours, and cover all wavelengths plants need. A 300W full-spectrum LED delivers the equivalent DLI of a 600W HPS while cutting electricity cost almost in half. Best choice for all crops at any scale.
🟡 HPS / HID High output, high heat
High-pressure sodium lights deliver very high PPFD output — historically the commercial standard for tomatoes, cucumbers, and cannabis. Efficiency runs 1.5–1.8 µmol/J. The primary tradeoffs are significant heat output (requiring ventilation investment) and electricity costs roughly double equivalent LED setups. Still used in large commercial operations where the initial capital cost of LED retrofitting is the barrier.
🔵 Fluorescent / T5 Seedlings & greens
T5 fluorescents produce gentle, diffuse light at low heat — ideal for seedlings, clones, microgreens, lettuce, and herbs. They cannot reach the PPFD needed for fruiting crops (typically maxing out at 200–300 µmol/m²/s without multiple units), making them unsuitable for tomatoes or cucumbers at flowering stage. Low initial cost and simple installation make them the best entry point for small-scale leafy green growing.
🌞 CMH / LEC Broadest spectrum
Ceramic Metal Halide lights produce a sunlight-like spectrum including UV wavelengths. UV enhances essential oil production in basil, mint, and rosemary, and increases terpene density in cannabis. Efficiency sits at 1.9–2.1 µmol/J — better than HPS, below modern LED. Excellent for herb specialists and any grower where aromatic quality matters more than raw yield per watt. Run 3–5°C cooler than HPS but still require ventilation investment.
Common light schedule mistakes to avoid
- Adjusting photoperiod and nutrients at the same time — when troubleshooting slow growth, change one variable at a time. If you switch from 18h to 16h and also change your nutrient solution on the same day, you can’t identify which change caused the result. Use the Nutrient Calculator independently from schedule changes.
- Relying on hours without checking DLI — 18 hours under a weak T5 fluorescent delivers roughly 8 mol/m²/day. 18 hours under a 400W LED at 700 µmol/m²/s delivers 45 mol/m²/day. Hours are the schedule; DLI is the actual dose. Measure PPFD with a PAR meter and use the DLI tab in this calculator.
- Switching to 12/12 too early in vegetative growth — for fruiting crops, plants need sufficient vegetative mass to support high yields. Switching cannabis to 12/12 before the plant has at least 4–6 nodes limits final yield significantly. Let the plant fill its intended canopy space before triggering flowering.
- Running lights 24/0 for extended periods — constant light causes chlorosis in most crops within 2–4 weeks due to disrupted carbohydrate metabolism. Even a 4-hour dark period (20/4) produces comparable or better growth than 24/0 while reducing electricity costs by 17%.
- Ignoring electricity cost at the planning stage — a single 1000W HPS running 18h/day at $0.15/kWh costs approximately $81/month. Switching to a 500W LED delivering equivalent DLI cuts that to around $40/month. Use the Cost tab to compare scenarios before purchasing fixtures.
Frequently Asked Questions
Related gardening tools — build a complete grow environment
Your light schedule is one part of a connected grow environment. After setting your photoperiod with this calculator, use the VPD Calculator to confirm your temperature and humidity match your growth stage — high VPD in a bright grow room causes heat stress that mimics light burn and is often misdiagnosed as excessive PPFD.
For hydroponic setups, check your EC/TDS reading alongside your light schedule — plants under higher DLI consume nutrients and water faster, requiring more frequent reservoir checks. The Hydroponic Nutrient Calculator will help you adjust solution strength as you move from vegetative to flowering stage, where phosphorus and potassium demands increase significantly.
Track whether your schedule changes are actually improving output with the Growth Rate Tracker — measure weekly stem height and compare against a baseline before you changed the schedule. And use the Yield Estimator at harvest to quantify the impact of DLI and photoperiod optimisation on your final results.