Every prepaid plan is sold on two numbers: a price and a validity. Operators put the price in large type and the validity in small type. That is the wrong way round, because the price alone cannot tell you whether a plan is good value.
The arithmetic
Cost per day is the plan price divided by its validity in days.
That is the whole formula. A plan costing 299 rupees for 28 days works out at about 10.68 rupees a day. A plan costing 579 rupees for 56 days works out at about 10.34 rupees a day. The second plan has a much bigger sticker price and is still the cheaper one.
The gap looks small per day. Over a year it is not: roughly 125 rupees, for the same service, purely from reading the second number.
Why longer validity usually wins
Two 28-day recharges cost you more than one 56-day recharge covering the same period, almost every time. Operators price longer commitments lower because a customer who has already paid for two months is a customer who will not switch next month.
This is why annual plans tend to have the lowest cost per day of anything on the shelf. If you can pay upfront, they are usually the best deal available.
Where this breaks down
Cost per day is not the only number that matters.
- A plan you cannot finish is not cheap. A very low cost per day on a plan with more data than you will ever use is money spent on nothing.
- Talktime top-ups have no validity at all, so they have no meaningful cost per day. They are a different kind of product.
- Coverage beats price. The lowest cost per day on an operator with a weak signal where you live is a bad deal, and no arithmetic fixes that.
How to use this
Do not compare recharges by price. Divide by the validity first, then compare.
PlanPick does this for every plan automatically, so you can sort by cost per day directly. The numbers in this post are worked examples to show the method — for what plans actually cost today, see all plans.