Photoperiod Timer Calculator

Plant class Day-neutral Long-day Short-day

Used for guidance and common timing cautions.

Growth phase Seedling Vegetative Flowering Custom

Helps match typical photoperiod targets.

Photoperiod preset 16/8 (common vegetative) 18/6 (fast vegetative) 14/10 (moderate) 12/12 (common flowering) 20/4 (high-light daylength) Custom hours

Preset sets light hours automatically.

Light hours (custom)

Used only when preset is Custom hours.

Anchor by Lights ON time Lights OFF time

Choose which time you already know.

Anchor time (24h)

Example: 06:00 or 20:30.

PPFD (optional)

Estimate DLI

DLI helps compare “light delivered” per day.

What you get

• Exact ON/OFF times with midnight rollover handling.
• Dark-window start and end times for light-leak checks.
• Optional Daily Light Integral estimate from PPFD.
• CSV export (server) and PDF export (browser).

Results appear above this form after submission.

Reset Calculate Photoperiod

Common photoperiod targets

These ranges vary by cultivar, intensity, and temperature. Use them as a starting point.

How the schedule is calculated

All times are computed to the minute. When OFF time passes midnight, the schedule rolls into the next day.

Steps for accurate timers

1. Select a plant class and growth phase for guidance.
2. Pick a photoperiod preset, or choose Custom hours.
3. Decide whether you know the ON time or OFF time.
4. Enter the anchor time in 24-hour format.
5. Optionally enable DLI and add PPFD for an estimate.
6. Press Calculate, then set your timer to the shown times.
7. Export CSV or PDF to keep the schedule handy.

Photoperiod targets by crop response

Photoperiod is the planned daily light window that controls growth signals and flowering cues. Long-day crops can bolt or flower as daylength rises; short-day crops often require long, uninterrupted nights; day-neutral crops respond more to total light and temperature. Common indoor targets include 14–16 hours for steady foliage, 16–18 hours for compact vegetative speed, and 12/12 when you want a consistent flowering trigger for sensitive varieties. Use the plant class selector to keep the “dark window” protected from light leaks.

Timer accuracy and midnight rollover

A timer schedule must stay consistent across weeks to avoid stress. This calculator anchors either the ON time or OFF time and computes the matching endpoint to the minute. If the OFF time passes midnight, the tool flags the rollover so you can set digital timers correctly. For example, 18.0 light hours anchored at 18:30 turns OFF at 12:30 the next day, and the dark window runs from 12:30 to 18:30.

Daily light integral as a planning metric

Photoperiod alone does not describe “light delivered.” When you enable the PPFD option, the calculator estimates DLI using DLI = PPFD × Light hours × 0.0036. Example: 450 µmol/m²/s for 16 hours gives 25.92 mol/m²/day, while 300 µmol/m²/s for 18 hours gives 19.44 mol/m²/day. This helps compare schedules when you adjust intensity or duration.

Using phases to reduce stretch and improve timing

Seedlings commonly use 14–18 hours with gentle intensity to limit stretch. Vegetative schedules often sit at 16–18 hours to build leaf area and shorten production time. Flowering schedules frequently use 12 hours light and 12 hours dark to stabilize bloom signals. When changing schedules, shift by 15–30 minutes per day to keep plants and timers aligned.

Example dataset for greenhouse planning

DLI values are in mol/m²/day. Adjust PPFD and hours together to meet crop targets.

1) What does “photoperiod” mean in practical terms?

It is the daily duration your lights are ON. The remaining hours are the dark window. Many crops use photoperiod as a signal for flowering, bolting, or vegetative growth.

2) Should I anchor the ON time or the OFF time?

Anchor the time you can keep most consistent. Many growers anchor ON time for routine, while short-day flowering often anchors OFF time to protect the night length.

3) Why is an uninterrupted dark window important?

Short-day crops may fail to flower if the night is interrupted by light leaks. Even brief exposures can alter signals, so use the dark start/end times to check leaks.

4) How accurate do timers need to be?

Minute-level consistency is usually enough. Avoid frequent daily drift. If you change schedules, move by 15–30 minutes per day to reduce plant stress and prevent mistakes.

5) What is PPFD and why does it affect DLI?

PPFD measures light intensity for photosynthesis in µmol/m²/s. DLI combines intensity and time, so longer photoperiods or higher PPFD increase the daily light delivered.

6) Can I run 20/4 safely for seedlings?

Some crops tolerate 20/4, but others benefit from more rest. Watch leaf temperature and hydration. If you see stress, reduce hours or intensity and stabilize the routine.

7) How do I use the exports in daily operation?

Export CSV to share schedules with staff or keep a log. Use the PDF for a quick reference near the timer panel. Recalculate whenever you change hours or anchor time.