While everyone is different, 83°-90° is a suggested temperature range for baptistry pool water. It does not need to be bathtub temp., but neither should it be chilly.

83° is probably the minimum temperature level for most average adults.

The baptistry water heating chart below will show the approximate # of hours required to raise the temperature of a given size pool by 30° 

NOTE: Chart below does not allow for heat loss so times given below will be less than the actual time required to heat a baptistry.




To Calculate the Number of Gallons in a Baptistry Pool
 
1) Measure the Inside Dimensions of the Baptistry
 
2) Round the measurements up to Nearest Whole Foot (Rounding Up gives a better margin of error.)

You can be more precise by rounding up on one figure and down on the other if your measurements allow. i.e. Pool measures 12’4 x 4’8, you can round each number to the closest whole number which gives 12x5 and is more accurate than rounding both numbers up. We are simply looking for “close” here. Since we are not allowing for ambient temperature loss, etc. the best we are going to do is a “guestimate” on the heating time, but it should be more than close enough for our purposes, especially since there are limited heater choices.We are also erring towards the high side.)
 
3) Multiply Length x Width x Water Depth (use 3*) x 7.5** = Gallons
 
*Most Fiberglass baptisteries are 42” deep. Actual Water Depth would be from 30” to 34” so we round up to 36” and use 3ft for the depth calculation.
**7.5 is the standard multiplier used to convert water volume from Cubic Feet to Gallons
 
Example Baptistry Dimensions

Actual Measurements are 12’4 long x 4’8wide

If we round up we use 13 x 5 x 3 x 7.5= 1462gals

If we rounded to the nearest foot: 12 x 5x 3 x 7.5 = 1350 gals

The difference in the 2 methods is 112 gallons translates into roughly 45 minutes heating time on 11kw heater or a bout 70 minutes on a 6kw heater.