How to Calculate Your Red Light Therapy Dose: Irradiance, Time, and Energy Explained
Published May 15, 2026. Updated October 2026.
Quick answer
Dose here means energy per square centimeter: J/cm² = mW/cm² x seconds / 1000. Irradiance comes from the spec sheet at a stated distance, and time is what you set. This is arithmetic about what a device emits, not a target and not a claim about outcomes. Follow your device manual for time and distance.
Tools
Do the arithmetic with the red light dose calculator, look up numbers in the irradiance by distance chart, see the session time table, or compare AWA devices.
The three numbers behind any dose calculation
People often say "dose" when they mean several different things: how long a session lasts, how bright the device is, or how many times a week they use it. In this guide, dose means energy delivered per square centimeter of surface, written as J/cm². It is built from two inputs you can look up and one you control.
- Irradiance in mW/cm². This is the power arriving on each square centimeter at a stated distance. It comes from the spec sheet and changes with distance. See the irradiance guide for how to read that number.
- Time in seconds. This is how long the light is on at that distance. It is the part you set.
- Energy in J/cm². This is the product of the first two, scaled for units.
Nothing in this guide says what a "right" J/cm² figure is. We cover the arithmetic and how to compare devices. Your device manual sets the session time and distance for that device, and a doctor is the right person to ask about your own situation. For how often AWA product pages list sessions, see our guide on how often to use red light therapy.
The formula, step by step
J/cm² = mW/cm² x seconds / 1000
The division by 1000 exists because irradiance is quoted in milliwatts, while a joule is a watt held for a second. One milliwatt for one second is one millijoule, and 1000 millijoules make one joule.
Four steps
- Find the irradiance in mW/cm² and note the distance it was stated at.
- Convert minutes to seconds by multiplying by 60.
- Multiply irradiance by seconds. The answer is in mJ/cm².
- Divide by 1000 to get J/cm².
Unit check
If a spec sheet lists W/cm² instead of mW/cm², multiply by 1000 first. If it lists total device watts (for example "300W"), you cannot do this calculation, because total power says nothing about how much of it lands on each square centimeter of your skin.
Irradiance x time grid (example numbers)
This grid shows how energy climbs as either input rises. Use it to build intuition, then replace the example irradiance values with the figure for your own device at your own distance.
| Example irradiance | 1 min | 5 min | 10 min | 15 min | 20 min |
|---|---|---|---|---|---|
| 20 mW/cm² | 1.2 | 6 | 12 | 18 | 24 |
| 40 mW/cm² | 2.4 | 12 | 24 | 36 | 48 |
| 60 mW/cm² | 3.6 | 18 | 36 | 54 | 72 |
| 80 mW/cm² | 4.8 | 24 | 48 | 72 | 96 |
| 100 mW/cm² | 6 | 30 | 60 | 90 | 120 |
Arithmetic example only. Every cell is J/cm² = mW/cm² x seconds / 1000. The irradiance values are round example numbers, not AWA specs and not a recommended dose.
Two patterns are worth noticing. Doubling the time doubles the energy. Doubling the irradiance does the same. That is why a device with a lower irradiance needs proportionally more time to deliver the same J/cm², and why comparing devices by LED count or total watts is misleading.
Worked examples with AWA stated numbers
The figures below use irradiance numbers published in our irradiance specs guide for the AWA FX300 panel and AWA FX500 panel. Each row is an arithmetic example, not advice on how long to run a session. For session times, use the manual and the product page.
| Device and distance | Stated irradiance (mW/cm²) | Time | Energy (J/cm²) |
|---|---|---|---|
| FX300 at 6 inches | 83.5 | 5 min (300 s) | 25.1 |
| FX300 at 6 inches | 83.5 | 10 min (600 s) | 50.1 |
| FX300 at 12 inches | 43.3 | 10 min (600 s) | 26.0 |
| FX500 at 6 inches | 94 | 5 min (300 s) | 28.2 |
| FX500 at 6 inches | 94 | 10 min (600 s) | 56.4 |
| FX500 at 12 inches | 73.8 | 10 min (600 s) | 44.3 |
Arithmetic examples. Irradiance figures are the ones listed in the AWA irradiance specs guide. The times are chosen to show the arithmetic and are not a session recommendation.
Example walk through
Arithmetic example: FX300 at 6 inches, 5 minutes. Irradiance 83.5 mW/cm². Time 5 x 60 = 300 seconds. Multiply: 83.5 x 300 = 25,050 mJ/cm². Divide by 1000: 25.05 J/cm², or 25.1 rounded.
Arithmetic example: the same panel at 12 inches for 10 minutes. Irradiance 43.3 mW/cm². Time 600 seconds. 43.3 x 600 = 25,980 mJ/cm². Divide by 1000: 25.98 J/cm², or 26.0 rounded. Notice that moving back to 12 inches and doubling the time lands near the energy of the 5 minute, 6 inch case.
These figures are calculations from stated specifications. They assume the whole area is lit at the stated irradiance, which real positioning will not match exactly.
Working backwards: time for an example target
You can rearrange the formula to find the time needed for a chosen energy figure: seconds = J/cm² x 1000 / mW/cm².
Arithmetic example with an invented target: suppose, purely for the maths, you pick 20 J/cm² as a target. The 20 is a made up number chosen to keep the arithmetic easy. It is not a recommendation.
- At 83.5 mW/cm² (FX300 stated figure at 6 inches): 20 x 1000 / 83.5 = 240 seconds, about 4.0 minutes.
- At 43.3 mW/cm² (FX300 stated figure at 12 inches): 20 x 1000 / 43.3 = 462 seconds, about 7.7 minutes.
The same target takes almost twice as long at double the distance on this panel. The ratio is not exactly two because irradiance does not fall in a perfectly neat pattern with distance, which is another reason a spec must name its distance.
Comparing two devices with hypothetical numbers
Here is a comparison using invented figures, with no connection to any real product. Device A is stated at 25 mW/cm² at its working distance. Device B is stated at 80 mW/cm² at its working distance. For a shared example figure of 24 J/cm²:
- Device A: 24 x 1000 / 25 = 960 seconds, which is 16 minutes.
- Device B: 24 x 1000 / 80 = 300 seconds, which is 5 minutes.
The calculation shows why irradiance, distance and time need to be read together. It does not tell you which session length is suitable for you. Only the manual for the device in front of you does that.
Common mistakes
- Using a spec stated at one distance while sitting at another. Always match the distance on the spec sheet.
- Skipping the divide by 1000 step, which inflates the answer a thousand times.
- Mixing minutes and seconds. The formula needs seconds.
- Comparing devices by LED count or total watts rather than irradiance at a stated distance.
- Using the maths as a target. The calculator only tells you what a device emits. It does not say what amount you should use.
- Assuming the same number applies across the whole body. A spec is usually given for one point or averaged over one area.
- Ignoring the manual. Session time and distance for your device belong to the manufacturer's instructions, not to a blog formula.
Before you use any light device: read the label, follow the device manual, and use eye protection as directed. Ask a doctor first if you have a medical condition, take medications, are pregnant, or have light sensitivity.
Related guides in this series
- What irradiance means on a spec sheet
- AWA FX300 and FX500 irradiance specs
- How long a session runs, by the manual
- Using a device beyond its manual
- 660nm vs 850nm wavelength guide
More in this series
- Red Light Therapy Dose Chart: Minutes by Distance
- Joules Per Session: How to Read Your Red Light Therapy Dose
Device pages
Read the full page and label for each device before you use it: AWA FX300 panel, AWA FX500 panel. You can also browse AWA panels.
Frequently asked questions
What is the red light therapy dose formula?
In this guide dose means energy per square centimeter. The arithmetic is J/cm² = mW/cm² x seconds / 1000. It describes what a device emits at a stated distance over a stated time.
Why do I divide by 1000?
Irradiance is quoted in milliwatts. One milliwatt for one second is one millijoule, and 1000 millijoules make one joule, so dividing by 1000 converts the answer to J/cm².
Can I use total device watts to calculate dose?
No. Total watts do not say how much power lands on each square centimeter. You need irradiance in mW/cm² at a stated distance.
Does this tell me how long to run a session?
No. The formula only converts irradiance and time into energy. Session time and distance for your device are set by its manual and product page.
Why does distance matter in the calculation?
Irradiance changes with distance, so a spec is only meaningful with its distance attached. Using a figure from one distance while sitting at another gives a wrong answer.
Sources
- FDA: General Wellness policy guidance (CDRH). Final guidance dated January 2026 that sets out the agency's compliance policy for general wellness products. It is a policy document, not a statement about any one device.
- FDA: Premarket Notification 510(k). Explains that a 510(k) is a premarket submission showing a device is substantially equivalent to a legally marketed device, and that a company must receive an FDA order clearing the device before marketing it in the U.S.
- FDA: Laser Products and Instruments. Describes FDA laser hazard classes I to IV, notes that higher classes carry warning labels, and states that LEDs are not subject to the federal laser product performance standard.
Regulatory note: select devices carry FDA 510(k) clearance for specific, limited uses. Most light devices are sold as general wellness products. Read each product page and label.
How we create this guide
AWA sells red light therapy devices. We write these guides to explain how devices work and how to choose and use one. This is general information, not medical advice. Read each product page and label. See our editorial standards.
