Ponds of irregular shape can be tricky to give accurate volume calculations for and often require some imaginative estimation. For an informal pond, imagine it as a rectangle and multiply the length x width x depth, then remove a percentage to allow for curves and planting shelves etc. if you are using metric measurements, a cubic metre is equal to 1,000 litres so convert your measurements into metres eg. 60cm = 0.6m and then multiply by 1000 to get a volume in litres. If you are measuring in feet and inches, a cubic foot is equal to 1,728 cubic inches, or 6.23 gallons. For circular ponds 3.142 multiplied by the radius squared will give you the surface area, which can then be multiplied by the depth to give volume before working out the capacity in the same way as above.
The best method of all is to time how long it takes to fill the pond with a hosepipe by first using a container of known volume to determine flow rate. If a hosepipe fills a 5 litre bucket in 4 minutes, a pond that fills in 80 minutes will contain 100 litres of water. It's easier than it sounds!