When the same water is poured into a rectangular tank with a smaller base, it forms a deeper layer, provided the tank is tall enough. Find the original water volume first, then divide by the new base area.
Use the two measurements along the bottom of each tank to find its base area. The third measurement gives the height available for the water.
- Find the first tank’s base area and multiply by the water depth.
- Find the second tank’s base area.
- Divide the same volume by the second base area and check the result fits.
Worked example | Pour the same water into a taller tank
Tank A has a base of 30 cm by 20 cm. The water is 6 cm deep, so its volume is 30 × 20 × 6 = 3,600 cm³.
All the water is poured into Tank B, with a 20 cm by 15 cm base and a height of 30 cm. Its base area is 300 cm². The new depth is 3,600 ÷ 300 = 12 cm. It fits below the rim.
Use the area change to check the depth
Tank A has a base area of 600 cm². Tank B’s 300 cm² base is half as large. Each centimetre of depth holds half as much water in Tank B, so the same water needs twice the depth.
You can also check by multiplying the new base area by the new depth. The product 300 × 12 gives the original 3,600 cm³.
Try a third base
The same 3,600 cm³ of water is poured into Tank C with a base of 30 cm by 15 cm. The tank is 20 cm tall. Find the water depth.
Answer. The base area is 450 cm². The water depth is 3,600 ÷ 450 = 8 cm.
Frequently asked questions
Why do I divide by base area?
In a rectangular tank, water volume equals base area multiplied by water depth. Division reverses that multiplication.
What if the calculated depth is higher than the tank?
The water will not all fit. Find the tank’s capacity by multiplying its base area by its full height, then subtract that capacity from the volume poured in to find the excess.
Official information
MOE, Primary Mathematics Syllabus (Primary 1 to 6), Primary Five, volume of liquid in a rectangular tank. Syllabus checked 11 November 2026. The examples are original.