To Determine Size Of Pond Liner 

Maximum Width =
Width + (2 times the depth) + 2 ft.
Maximum Length =
Length + (2 times the depth) 2 ft.

For example:
4 ft. width x 3 ft. depth (18 in. x 2) + 2 ft. = 9 ft. maximum width
6 ft. length x 3 ft. depth (18 in. x 2) + 2 ft. = 11 ft. maximum width
The liner must be at least 9 ft. by 11 ft.
Length = L+2D+2 ft. Width = W+2D+ 2 ft.  

To Determine Water Volume In Gallons  

Formula Hint:
When measuring, round to the nearest whole foot to work the formula.
Length x Width x Depth x amount indicated below for the shape of pond.

For example:
(Using dimensions of pond above) 4 ft. W x 6 ft. L x 2 ft. D (18 in. rounded to 2 ft.) x 6.5 = 312 Gallons  

To Determine Pump Size For A Waterfall  

First, determine the rise by measuring the distance from the pond water level to the top of the fall in ft. Then, for each 10 ft. of hose from the pump to the fall, add 1 ft. to the rise. Once you know the total rise of the waterfall (the actual rise + the rise from the length of the hose), look at the sizing chart found on the pump box.

For example:
A waterfall 12 in. wide with a rise of 3 ft. and a hose run of 20 ft. would require a pump capable of pumping 1200 gallons of water per hour at 5 ft. of rise.  

Width of waterfall (in inches) x 100 gallons per hour
Rise (in ft.) + Hose Rise (10 ft. hose length = 1 ft.) = Total waterfall rise 

Flow Rate For Biological Or Bio-Filter

The filter should pump all the water every 1-1/2 to 2 hours.
Flow per hour through a biological filter is 1/2 the volume of the pond.

For example:
A 1,000 gallon pond will require a biological filter than can pump 500 gallons per hour.