In short: %UVT tells you what share of UV light at 254 nm makes it through 1 cm of your water. It's calculated from absorbance with %UVT = 100 × 10⁻ᴬ, where A is the absorbance normalized to a 1 cm path length. Well-treated drinking water is often 90–98% UVT; raw or high-organic water can be much lower.
This is a unit-conversion tool for a lab reading, not a substitute for your UV system's validated dose curve — check the manufacturer's documentation for actual disinfection sizing.
How %UVT is calculated
UV transmittance follows the Beer–Lambert law: dissolved organic matter and other compounds absorb UV light in proportion to their concentration and the distance the light travels through the water. A spectrophotometer reports that absorption as absorbance (A254) for whatever cell it used, so the formula first normalizes to a standard 1 cm path length, then converts to a percentage:
%UVT = 100 × 10−A254 A254 = 2 − log₁₀(%UVT)
If your reading came from a cell other than 1 cm, divide the measured absorbance by the path length in centimeters first: A254 = A_measured ÷ L.
Worked example. A 1 cm cell reads an absorbance of 0.046 at 254 nm. %UVT = 100 × 10⁻⁰·⁰⁴⁶ ≈ 89.9%, meaning about 89.9% of the UV light entering that first centimeter of water makes it through, and roughly 10.1% is absorbed.
Longer-cell example. The same lab instead reads 0.350 using a 5 cm cell. Normalized to 1 cm, that's A254 = 0.350 ÷ 5 = 0.070, giving %UVT = 100 × 10⁻⁰·⁰⁷⁰ ≈ 85.1% — noticeably lower than the first sample, and a reminder that skipping the path-length correction would badly overstate the water's transmittance.
%UVT to absorbance quick reference
All values are at the standard 1 cm path length. Use this to sanity-check a manual calculation or to see roughly what absorbance a target %UVT implies.
| %UVT | A254 (1/cm) | Transmittance (T) |
|---|---|---|
| 99% | 0.0044 | 0.990 |
| 98% | 0.0088 | 0.980 |
| 97% | 0.0132 | 0.970 |
| 96% | 0.0177 | 0.960 |
| 95% | 0.0223 | 0.950 |
| 94% | 0.0269 | 0.940 |
| 92% | 0.0362 | 0.920 |
| 90% | 0.0458 | 0.900 |
| 85% | 0.0706 | 0.850 |
| 80% | 0.0969 | 0.800 |
| 75% | 0.1249 | 0.750 |
| 70% | 0.1549 | 0.700 |
What else affects the result
Path length compounds fast. Because absorbance is linear with distance, a water with A254 = 0.10 per centimeter transmits about 79% of UV light through a 1 cm layer, but only about 32% through 5 cm and roughly 10% through a 10 cm layer. UV reactor sleeves and lamp spacing are designed around this, which is why a sample's raw transmittance says less about disinfection performance than its interaction with the specific reactor geometry.
| Path length | Transmittance |
|---|---|
| 1 cm | 79.4% |
| 2 cm | 63.1% |
| 5 cm | 31.6% |
| 10 cm | 10.0% |
What actually absorbs UV. Dissolved organic carbon, iron, and certain industrial or agricultural compounds are the usual causes of low %UVT; none of these necessarily show up as turbidity or color you can see. Wavelength matters too — this calculator and the standard %UVT convention are both specific to 254 nm, the wavelength most low-pressure UV disinfection lamps emit.
To turn a %UVT figure into an actual disinfection dose, pair it with your system's lamp intensity and contact time using the UV dose calculator.
Frequently asked questions
What is UV transmittance (%UVT)?
UV transmittance, or %UVT, is the percentage of UV light at 254 nanometers that passes through 1 centimeter of water. It is the standard way water treatment engineers describe how much UV light a sample lets through, and it directly affects how large a UV disinfection system needs to be.
How do I convert absorbance to %UVT?
Use %UVT = 100 × 10^(−A254), where A254 is the water's absorbance at 254 nm normalized to a 1 cm path length. To go the other way, A254 = 2 − log10(%UVT).
What if my spectrophotometer used a cuvette longer than 1 cm?
Divide the measured absorbance by the cell's path length in centimeters before applying the %UVT formula. Absorbance scales linearly with path length, so a reading of 0.35 in a 5 cm cell is equivalent to 0.07 per centimeter.
What %UVT is considered good for UV disinfection?
Well-treated municipal drinking water is typically in the 90–98% UVT range. Raw surface water, groundwater with organics or iron, and wastewater effluent are often lower, sometimes well under 75%. Always check your UV system's validated dose curve against your site's actual %UVT rather than assuming a typical value.
Why does low %UVT matter for sizing a UV system?
Water that absorbs more UV light delivers less intensity to organisms at a given distance from the lamp, so a lower %UVT reduces the dose the system can achieve at a set flow rate. UV systems are validated at a minimum %UVT, and water below that point needs a lower flow rate, more or stronger lamps, or pretreatment to raise the %UVT.
Is %UVT the same as turbidity or water clarity?
No. %UVT specifically measures how much dissolved organic matter and other UV-absorbing compounds attenuate light at 254 nm. Turbidity measures light scattering from suspended particles. A sample can be low in turbidity yet still have a low %UVT because of dissolved color or organics, so the two should be measured separately.
Sources & references
- U.S. Environmental Protection Agency (EPA). Ultraviolet Disinfection Guidance Manual for the Final Long Term 2 Enhanced Surface Water Treatment Rule (2006).
- APHA/AWWA/WEF. Standard Methods for the Examination of Water and Wastewater, Method 5910 (UV-Absorbing Organic Constituents).
- NSF International. NSF/ANSI Standard 55: Ultraviolet Microbiological Water Treatment Systems.
Sarah Bennett, MS
Water Quality & Sustainability Specialist · Portland, Oregon
Environmental scientist (MS, Water Resources) with 9+ years in residential water quality, treatment, and usage efficiency, and 150+ completed assessments. Sarah writes and reviews the treatment, usage, and cost calculators on watercalculator.us. Read full bio
MS Environmental Science, Water Resources (2018) · BS Environmental Engineering (2016)