What the Blaney-Criddle method calculates
This calculator estimates reference crop evapotranspiration, written ETo: the depth of water that a large, well-watered surface of grass 8 to 15 cm tall would lose to evaporation and transpiration in a day. ETo describes the climate, not any one plant. To get water use for an actual lawn or crop, ETo is multiplied by a crop coefficient (Kc) specific to that plant and growth stage — a separate step covered by the crop evapotranspiration calculator.
The Blaney-Criddle equation is one of several ways to estimate ETo. Its appeal is that it needs only air temperature, plus a location-based daylight factor, when humidity, wind and sunshine records aren't available. FAO's irrigation water needs training manual describes it as simple but not very accurate: a rough, order-of-magnitude figure rather than a precise measurement.
| Climate zone | Low temp. (<15°C / 59°F) | Medium (15–25°C / 59–77°F) | High (>25°C / 77°F) |
|---|---|---|---|
| Desert / arid | 4–6 mm/day | 7–8 mm/day | 9–10 mm/day |
| Semi-arid | 4–5 mm/day | 6–7 mm/day | 8–9 mm/day |
| (Moist) sub-humid | 3–4 mm/day | 5–6 mm/day | 7–8 mm/day |
| Humid | 1–2 mm/day | 3–4 mm/day | 5–6 mm/day |
These ranges are a sanity check, not a substitute for the calculation: they show that a temperature alone doesn't fix ETo, because a hot desert and a hot humid region at the same temperature can differ by several mm/day. Aridity and cloudiness, both left out of the plain Blaney-Criddle formula, explain most of that gap. That gap is also why the calculator's accuracy note above always accompanies the result rather than only appearing for unusual inputs.
How this calculator works
The calculator applies the Blaney-Criddle formula as presented in FAO's Irrigation Water Management Training Manual No. 3, using your mean temperature and the daylight percentage for your latitude and month.
1. The Blaney-Criddle formula
Reference ET (ETo)
ETo = p × (0.46 × Tmean + 8)
ETo is in mm/day, Tmean is the mean temperature in °C, and p is the mean daily percentage of annual daytime hours (as a decimal, e.g. 0.29 for 29%). The result is meant as a monthly average, not a single day's forecast.
2. Finding p, the daylight percentage
p depends only on latitude and month — it's a measure of how long the days are, not the weather. FAO's Table 4 gives p in 5-degree latitude bands from the equator to 60°, for both hemispheres (a Southern Hemisphere month uses the Northern Hemisphere value six months away, since the seasons are reversed). The calculator looks this value up for you when you choose a location; you can also enter your own p value directly if you have one from another source.
| Latitude | Jan / Jul | Feb / Aug | Mar / Sep | Apr / Oct | May / Nov | Jun / Dec | Jul / Jan | Aug / Feb | Sep / Mar | Oct / Apr | Nov / May | Dec / Jun |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 60° | .15 | .20 | .26 | .32 | .38 | .41 | .40 | .34 | .28 | .22 | .17 | .13 |
| 55° | .17 | .21 | .26 | .32 | .36 | .39 | .38 | .33 | .28 | .23 | .18 | .16 |
| 50° | .19 | .23 | .27 | .31 | .34 | .36 | .35 | .32 | .28 | .24 | .20 | .18 |
| 45° | .20 | .23 | .27 | .30 | .34 | .35 | .34 | .32 | .28 | .24 | .21 | .20 |
| 40° | .22 | .24 | .27 | .30 | .32 | .34 | .33 | .31 | .28 | .25 | .22 | .21 |
| 35° | .23 | .25 | .27 | .29 | .31 | .32 | .32 | .30 | .28 | .25 | .23 | .22 |
| 30° | .24 | .25 | .27 | .29 | .31 | .32 | .31 | .30 | .28 | .26 | .24 | .23 |
| 25° | .24 | .26 | .27 | .29 | .30 | .31 | .31 | .29 | .28 | .26 | .25 | .24 |
| 20° | .25 | .26 | .27 | .28 | .29 | .30 | .30 | .29 | .28 | .26 | .25 | .25 |
| 15° | .26 | .26 | .27 | .28 | .29 | .29 | .29 | .28 | .28 | .27 | .26 | .25 |
| 10° | .26 | .27 | .27 | .28 | .28 | .29 | .29 | .28 | .28 | .27 | .26 | .26 |
| 5° | .27 | .27 | .27 | .28 | .28 | .28 | .28 | .28 | .28 | .27 | .27 | .27 |
| 0° (Equator) | .27 | .27 | .27 | .27 | .27 | .27 | .27 | .27 | .27 | .27 | .27 | .27 |
Read a Northern Hemisphere value straight from its month column. For a Southern Hemisphere site, use the second month label in each header — for example, at 35°S in January, read the "Jul / Jan" column, which gives .32.
3. Unit conversions
Temperature
°C = (°F − 32) × 5 ⁄ 9
The formula itself always runs in Celsius. In US mode, the calculator converts your °F entry to °C first, then converts the mm/day result back to inches/day for display.
Depth
Inches/day = mm/day ÷ 25.4
Rounding
Your entered temperature and daylight percentage are shown back exactly as typed. ETo is shown to 1 decimal place in mm/day or 2 decimal places in inches/day, matching the precision used in the underlying calculation, and is always labeled "about" because of the method's documented error range.
When this calculator isn't the right tool
Blaney-Criddle is a temperature-only fallback. Use a different tool when:
- You have humidity, wind and sunshine or solar radiation data. FAO recommends the Penman-Monteith method wherever that data is available; see the reference ET calculator.
- You only have daily maximum and minimum temperatures and want a method built specifically around that data. See the Hargreaves ET calculator.
- You have pan evaporation readings instead of temperature. See the pan evaporation to ET calculator.
- You want actual crop or lawn water use, not the grass reference value. See the crop evapotranspiration calculator.
All of these live alongside this tool in the scheduling and ET calculators.
Blaney-Criddle calculation example
A site at 35°N has a mean April temperature of 75°F. From the table, 35°N in April gives p = 0.29.
- Convert to Celsius: (75 − 32) × 5 ⁄ 9 = 23.9°C.
- Apply the formula: ETo = 0.29 × (0.46 × 23.9 + 8) = 0.29 × 18.99 = 5.51 mm/day.
- Convert to inches: 5.51 ÷ 25.4 = about 0.22 in/day.
The same location in metric
Same site, 35°N, April, but with a mean temperature of 24.5°C and the same p = 0.29 (FAO's own worked example in Training Manual No. 3, based on a mean of 29.5°C max and 19.4°C min):
- Apply the formula: ETo = 0.29 × (0.46 × 24.5 + 8) = 0.29 × 19.27 = 5.59 mm/day, shown as about 5.6 mm/day.
- Per week: 5.5883 × 7 ≈ about 39.1 mm/week.
- Per 30-day month: 5.5883 × 30 ≈ about 167.6 mm/month.
The small gap between 5.51 and 5.59 mm/day comes entirely from the 0.6°C difference between 75°F converted (23.9°C) and the metric example's 24.5°C — the formula itself behaves identically in both unit systems.
Using ETo for irrigation scheduling
ETo on its own doesn't tell you how much to water; it's an input to that decision. The usual next steps are to multiply ETo by a crop coefficient to get actual crop water use, then subtract effective rainfall to find the net irrigation requirement for the period.
Getting a reliable mean temperature
Look for a monthly average from a nearby weather station, agricultural extension network, or airport record, rather than checking today's forecast. If you only have daily highs and lows, average them across the whole month rather than using a single day's max and min — the Blaney-Criddle formula is calibrated against monthly means, and a single unusually hot or cold day will skew the estimate.
Turning ETo into a crop or lawn water need
Multiply the ETo from this calculator by the crop coefficient (Kc) for your plant and growth stage to get ETcrop, the actual expected water use. The crop coefficient table lists typical Kc values, and the crop evapotranspiration calculator does this step for you. From there, subtracting effective rainfall with the effective rainfall calculator gives the net amount your irrigation system needs to supply, which the irrigation water requirement calculator can carry through to a schedule.
Common mistakes
- Using a single day's temperature. The method assumes a monthly mean; a heat wave or cold snap will distort a one-day estimate.
- Guessing p instead of looking it up. p depends only on latitude and month, so it's worth getting right — a wrong hemisphere or a latitude band far from your actual site will shift the result more than most people expect.
- Treating ETo as the amount to irrigate. ETo is a reference value for grass, before any crop coefficient or rainfall is applied.
- Applying it to a single hot, dry, windy day and expecting accuracy. That's exactly the condition where FAO's guidance says the method underestimates the most.
Check for a local, calibrated method first. Where a state or regional agency publishes locally calibrated ET coefficients or an ET network (such as a state AgriMet or CIMIS-style service), those figures reflect local wind, humidity and elevation better than the plain Blaney-Criddle formula and should generally be used instead when available.
What changes a Blaney-Criddle estimate
Because the formula only takes temperature and daylight length, several real climate factors change actual ETo without being reflected in this calculator's result at all.
| Factor | Effect on actual ETo | What to do about it |
|---|---|---|
| Wind speed | Higher wind increases actual evapotranspiration beyond the temperature-only estimate. | In windy areas, treat this calculator's result as a likely underestimate. |
| Relative humidity | Low humidity increases actual ETo; high humidity suppresses it. | In humid climates, expect this calculator to overestimate. |
| Sunshine / cloudiness | More sunshine hours increase actual ETo for a given temperature. | Cloudy, humid climates combine both effects, which is why FAO's guidance puts the overestimate there at up to 40%. |
| Elevation | Published calibration studies have found elevation-related bias in Blaney-Criddle ET estimates at higher-elevation sites. | Use a local, elevation-adjusted coefficient if your agency publishes one. |
| Latitude/month accuracy | p changes with both, and the table only covers whole 5° latitude bands. | Choose the closest latitude band; the difference between adjacent bands is usually small. |
A peer-reviewed calibration study of the FAO Blaney-Criddle formula against lysimeter-measured alfalfa evapotranspiration in Idaho found that adding an elevation correction reduced scatter in the estimates, and that local calibration against measured wind, humidity and radiation brought the method much closer to Penman-based results. That's the general pattern with Blaney-Criddle: the plain formula is a starting point, and local calibration closes most of the gap.
Frequently asked questions
What does the Blaney-Criddle method actually calculate?
It estimates reference crop evapotranspiration (ETo): the water loss from a well-watered grass surface, in millimeters or inches per day. ETo is a climate number, not a prediction for a specific plant. To get water use for an actual crop or lawn, ETo is multiplied by a crop coefficient (Kc) for that plant and growth stage.
Where do I find the daylight percentage (p) for my location?
Use the location option above the result: pick your hemisphere, the latitude band closest to your site, and the month. The calculator looks up p from the FAO's published table of mean daily daytime-hour percentages by latitude and month. If you already have a p value from another source, switch to "Enter my own percentage" instead.
How accurate is the Blaney-Criddle method?
FAO's Training Manual No. 3 describes it as a rough, order-of-magnitude estimate, not a precise measurement. In windy, dry, sunny climates it can underestimate ETo by up to about 60 percent, and in calm, humid, cloudy climates it can overestimate ETo by up to about 40 percent. Methods that also use humidity, wind and sunshine data, such as FAO Penman-Monteith, are more accurate when that data is available.
Can I enter a single day's temperature instead of a monthly average?
The formula was built and tested using mean monthly temperature, and FAO guidance recommends applying it over periods of a month or longer. A single day's temperature can swing well above or below the monthly mean, so plugging it in will shift the result away from what the method is calibrated to produce. Use the average of a full month, or at least several days, for a more reliable estimate.
Why did my result show 0.0 instead of a negative number?
Below about -17°C (1°F) mean temperature, the formula's bracket term turns negative, which would imply negative evapotranspiration. That isn't physically meaningful, so the calculator shows 0.0 and flags the input instead of displaying a negative result. In practice, evapotranspiration is negligible at these temperatures because the ground is typically frozen.
Is Blaney-Criddle still used today?
Yes, mainly as a fallback when only temperature records are available. FAO's 1998 Penman-Monteith guidelines (Irrigation and Drainage Paper 56) established Penman-Monteith as the preferred reference method wherever humidity, wind and sunshine or radiation data can be obtained, but temperature-only methods like Blaney-Criddle remain useful for remote sites, historical records, or quick planning estimates.
Do I need a weather station to use this calculator?
You need a mean temperature for the period you're estimating, which can come from a nearby station, an agricultural extension network, or a weather history site. You do not need humidity, wind or sunshine readings. That's the entire appeal of the Blaney-Criddle method: it trades accuracy for working with temperature data alone.
Does this calculator give me my crop's actual water use?
No. It gives ETo, the reference value for a grass surface. Actual crop water use (ETc) is ETo multiplied by a crop coefficient (Kc) that depends on the plant and its growth stage. The crop evapotranspiration calculator carries that next step forward.
See the crop evapotranspiration calculator.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Formula, unit conversions and both worked examples verified by hand, including a cross-check against FAO's own published worked example.
- The daylight-percentage table is reproduced from FAO's published source table, not estimated or interpolated.
- Accuracy limits are quoted directly from FAO guidance, not estimated.
Sources
- Brouwer, C. and Heibloem, M. FAO Irrigation Water Management Training Manual No. 3. Irrigation Water Needs, Chapter 3: Crop Water Needs (1986).
- Doorenbos, J. and Pruitt, W.O. FAO Irrigation and Drainage Paper No. 24. Crop Water Requirements (1977).
- Allen, R.G., Pereira, L.S., Raes, D. and Smith, M. FAO Irrigation and Drainage Paper No. 56. Crop Evapotranspiration: Chapter 2, the FAO Penman-Monteith Equation (1998).
- Allen, R.G. and Pruitt, W.O. Rational Use of the FAO Blaney-Criddle Formula. Journal of Irrigation and Drainage Engineering, 112(2) (1986).