What size liner do I need?

The formula for sizing a pond liner is
POND LENGTH + (2 TIMES DEPTH) + 2' = LINER LENGTH
POND WIDTH + (2 TIMES DEPTH) + 2'= LINER WIDTH
Example:
A 12' long, 7' wide and 3' deep pond is...
12' + (2 * 3) +2' = 20' Long
7' + (2 * 3) + 2' = 15' Wide

How many gallons is my pond?

The formula for determining the number of gallons is:
FOR SQUARE AND RECTANGULAR PONDS
Length * Width * Depth * 7.5 = Gallons

FOR ROUND PONDS
RADIUS * RADIUS * 3.14 * 7.485 = Gallons
Example:
12 * 12 * 3.14 * 7.485 = 3,384 Gallons

Consider Adding UnderGuard Geotextile Underlayment

UnderGuard Geotextile Underlayment™
The biggest contributors to Koi pond, water garden and waterfall failures are poor, unstable soil conditions. Nonwoven geotextile fabrics are perfect for stabilizing soil, preventing rocks and roots from penetrating pond liner and for creating ’rock pads’ to cushion large rocks and boulders placed in the pond on top of the liner.
Underlayment is EASY to install
Underlayment is manufactured in 5’, 10’ 15’ widths and will ship to you in one or more pieces equal to the total square footage of the pond liner in liner/geo combo. Simply lay the pieces next to each other and install the pond liner. Use any extra underlayment to create pads for large rocks and boulders placed inside your pond.