Mobile Data Rollover Calculator

Why rollover data feels like a bonus (and sometimes isn't)

Many mobile plans promise "data rollover" — the unused gigabytes from last month get added to this month. It sounds like free data, and when the math is right, it is. But rollover almost always comes with a cap: your plan may allow at most 25 GB to carry over, no matter how much you actually saved. Anything above the cap simply vanishes at the end of the billing cycle. So the real question is never "how much did I save?" — it is "how much of it survives, and how much allowance do I truly have this month?" This calculator answers exactly that.

How the rollover math works

The calculator takes four inputs: your base monthly allowance, last month's unused data, your plan's rollover cap, and this month's expected usage. It first clips the rollover to the cap — the smaller of saved and cap — and treats the rest as expired. Your true allowance is base plus rolled-over. Then it subtracts this month's expected usage and shows the remainder, the percentage used, and a verdict: Comfortable (under 80% used), Tight but workable (80–100%), or Over your allowance (above 100%). Rollover can't be negative, and a cap of zero means your plan has no rollover at all.

Worked example

Suppose your plan gives you 50 GB a month and your rollover cap is 25 GB. Last month you used 40 GB, so 10 GB is available to roll. Enter 50, 10, 25, and this month's expected usage of 45 GB. The calculator finds 10 GB rolls over (all of it, since 10 is below the 25 cap), 0 GB expires, and your true allowance is 60 GB. After 45 GB of usage, 15 GB remains — 75% used, verdict Comfortable. Now imagine you had saved 30 GB instead: only 25 GB rolls over, 5 GB expires, and your true allowance is 75 GB. With 55 GB of expected usage you'd still be Comfortable — but you'd also see a warning that 5 GB evaporated.

Practical tips to stop wasting rollover data

Check your plan's rollover cap before celebrating a light month — the data above the cap is gone forever, not saved for later. If rollover expires every single month, your plan is probably too big; downsizing to match your real average usage usually saves more than the rollover ever did. Some carriers pool rolled-over data with base data, others use base data first — this calculator pools them together, which is the safer estimate. And remember that heavy one-off months are exactly what rollover is for: the extra cushion is there so you don't pay overage fees, so don't treat it as a reason to upgrade.