Short answer
Multiply the box's length, breadth and height in centimetres, then divide by 5000, which most Indian couriers use. A 30 × 20 × 15 cm box is 1.8 kg. The courier bills the larger of that and the real weight. The free volumetric weight calculator on this site does it for you.
Free tools for this
Why couriers use it
A truck or a plane fills up with space before it fills up with weight. So couriers charge for the room a parcel takes as well as for how heavy it is. That room is turned into a weight, called the volumetric weight.
The formula
Volumetric weight in kg = length × breadth × height in cm ÷ divisor. Most Indian couriers use a divisor of 5000. Some use 4000 or 6000, so check your rate card.
Try it now: Volumetric Weight Calculator → · Shopify Profit Margin and Fees Calculator →
A worked example
A box of 30 × 20 × 15 cm is 9,000 cubic centimetres. Divided by 5,000, that is 1.8 kg. If what is inside weighs 0.8 kg, you are billed for 1.8 kg, the larger. Many couriers also bill in steps of 500 g, so this is likely to be billed as 2 kg.
Where the money leaks
A small, light product shipped in a large box is billed as a heavy one. Across thousands of orders the extra weight band can cost more than the product's margin. Odd box sizes and too much filler are the usual causes.
How to bring it down
Use the smallest box the product fits in. Keep a few box sizes so each product gets one that suits it, rather than one size for everything.
Pad with less, and use mailers or poly bags for soft goods. Then compare your couriers on the same parcel, because the divisor and the slab size both change the bill.
Put it in your numbers
Shipping is one line in what an order really earns. The Shopify profit calculator on this site includes it alongside payment fees and returns, so you can see what a bigger box does to the margin.