SIM only deals with data rollover UK October 2026: how iD Mobile and O2 compare

Last updated: Friday 9 October 2026 (Europe/London).
Next update: when iD Mobile or O2 change their rollover rules or the prices below.
By: Switch Editorial Team, Switch Squid Ltd

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As of 9 October 2026, iD Mobile’s 60GB SIM only plan costs £10 a month on a 30-day plan (£120 over 12 months), and iD says unused data rolls over into the next month at no extra cost. O2 includes rollover on its Plus and Ultimate plans, but not on Classic, so the plan name matters as much as the price if your data use changes from month to month. SwitchSquid sets out how each network’s rollover works, with one worked example, so you can judge what it is worth to you; we may earn a commission if you buy through our links.

Sources: iD Mobile — data rollover, iD Mobile — SIM only plans, iD Mobile — pay monthly terms, O2 — data rollover and Smarty — 16GB plan.

What data rollover means in practice

Most SIM only plans give you a fixed amount of data each month. If you use 18GB of a 30GB allowance, the other 12GB normally vanishes when your billing month ends. Data rollover changes that: some or all of the leftover amount is added to the following month, so a quiet month can pay you back during a busy one.

It is easiest to think of rollover as a small safety margin rather than a way of saving money. You still pay the same monthly price. What you gain is protection against the month when you travel, work away from home wifi, stream more over the winter evenings or share your hotspot with a laptop. If your usage is steady and you always finish with plenty to spare, a cheaper plan with a smaller allowance may suit you better than a larger plan with rollover.

Rollover also differs from paying for a bigger plan “just in case”. A 100GB plan costs more every month, whether or not you need the extra. Rollover only comes into play in the months you under-use, and it only matters if the next month is heavier.

SIM only deals with data rollover compared (checked 9 October 2026)

The table below uses iD Mobile plans, because iD lists its plans, prices and rollover feature on its own pages and we could compare every row against its SIM only listing when we checked. The yearly cost is simply the monthly price multiplied by 12 (or by 24 for the two-year plan, shown as a 12-month equivalent).

iD Mobile plan	Data a month	Contract	Price a month	Cost over 12 months	Rollover
30GB SIM only	30GB	30-day plan	£8	£96	Unused data added to next month
60GB SIM only	60GB	30-day plan	£10	£120	Unused data added to next month
100GB SIM only	100GB	30-day plan	£12	£144	Unused data added to next month
50GB SIM only	50GB	12 months	£8	£96	Unused data added to next month
15GB SIM only	15GB	24 months	£6	£72 (£144 over 24 months)	Unused data added to next month

All five plans include unlimited minutes and texts. The 60GB plan at £10 is our main example because it gives a large allowance on a plan you can leave with 30 days’ notice, and the 50GB 12-month plan at £8 is the lower monthly price if you are happy to commit for a year. iD’s unlimited plan is not in the table because an unlimited allowance has nothing left over to roll.

How the rules differ: iD Mobile, O2 and others

iD Mobile. iD describes data rollover as a free feature: unused data is added to your next month’s allowance, and the leftover data is used first. Its SIM only page adds that data rolls over once and expires after one month if you do not use it. Its pay monthly terms add some important details. Only your monthly inclusive data counts, so add-ons and reward allowances are not rolled over. Unlimited data plans are not eligible. If you move to a different tariff, unused rolled-over data is lost. The terms also say the rollover period is set out in your Tariff Summary, so it is worth reading that line when your confirmation arrives.

O2. O2 includes rollover on its Plus and Ultimate plans for new and upgrading customers. Up to 100% of the unused part of your main allowance can roll over, so on a 50GB plan you could have up to 100GB the following month. Your main data is used first and the rolled-over data second, and rolled-over data lasts for one billing cycle only. It can only be used in the UK. Unlimited plans are excluded, and if you move from a Plus or Ultimate plan to a Classic plan you lose rollover as an ongoing benefit. So check the plan name before you rely on rollover at O2.

Smarty. Smarty’s plan page lists one-month rolling plans with no annual price rises and no credit check, but it does not list data rollover. Its 16GB plan at £8 a month can suit people with very steady usage who value the freedom to change plan each month.

The order in which each network spends your data is the detail people miss. iD says leftover data is used first, which means rolled data is never wasted on a normal month. O2 spends your main allowance first, so rolled-over data only matters once you have used the whole allowance for that month, and it disappears at the end of the next cycle if you do not get that far.

Is rollover worth paying for? A worked example

Take a typical year on a 30GB plan. Say you use 22GB in September, leaving 8GB. On iD’s description, that 8GB is added to October, giving you 38GB. If October is a heavy month because you are away and rely on mobile data, you may use 36GB and finish without paying for extra data or buying an add-on. Without rollover, you would have run out at 30GB.

Now think about the opposite case. If you use 22GB every month, rollover will never change your bill, and the same money could buy a 25GB plan if one is available at a lower price. In other words, rollover is a form of insurance, and like all insurance it is worth most when your use is uneven.

A rule of thumb: look at your last six months of usage in your network app. If the highest month is more than about a third above your average, rollover can help. If every month is within a few gigabytes of the others, choose the plan size that fits and treat rollover as a bonus.

Choosing a plan size when you have rollover

Because leftover data carries forward, you can choose a plan slightly closer to your average use rather than your worst month. For example, someone who averages 25GB but occasionally reaches 40GB might choose a 30GB plan on iD and let rollover cover the busy month. Someone who regularly goes beyond 50GB is usually better off on a larger plan, because rollover moves a surplus forward but cannot create data you have not paid for.

• Light use (under 10GB a month): a small plan is enough; rollover is a minor extra.
• Medium use (15GB to 40GB): iD’s 30GB or 50GB plans with rollover cover most months.
• Heavy use or hotspot use (50GB and above): consider the 60GB or 100GB plan, since rollover helps but cannot replace a bigger allowance.
• Travel in Europe: iD’s terms say you can use up to 30GB of your monthly allowance while roaming in its Roam Free destinations, and any rolled-over data counts towards that limit.

How to check rollover terms before you sign

The marketing line “data rollover” can hide several different rules, so check four things on the plan page and in your confirmation:

1. Is rollover included in this exact plan? At O2 it depends on Plus or Ultimate versus Classic. At iD it is described as included on its plans, but the Tariff Summary is where the rollover period is stated.
2. Which data is used first? This decides whether rolled data is used in a normal month or kept as back-up.
3. How long does rolled data last? iD says rolled data expires after one month and O2 says one billing cycle. Look for the same wording from any other provider.
4. What cancels it? Changing plan can wipe rolled data at iD, and O2 removes the ongoing benefit if you move to a plan that is not Plus or Ultimate.

It is also worth reading our guide to rolling versus 18-month SIM plans if you are weighing a 30-day plan against a longer contract, and SIM only deals with no price rises if a steady monthly price matters more to you than extra data.

Switching without losing your number

You can move your number to a new SIM using a PAC code. Text PAC to 65075 from your current phone and the code arrives by text (some providers also ask for your date of birth in the message). Our text-to-switch guide explains the steps. Using your PAC ends your old agreement, so check whether your current plan has any remaining months to pay before you request it; a 30-day plan usually has none. If your phone supports it, an eSIM can be activated within minutes, and our eSIM versus physical SIM guide sets out the differences.

Remember that unused data on your old plan will not move with you. If you have a large rolled-over balance, use it before you switch, or time the switch for just after a heavy month.

Fit summary

• Good for iD Mobile 60GB at £10: people who want a large allowance on a plan they can leave with 30 days’ notice, with leftover data carried forward.
• Good for iD Mobile 50GB at £8 for 12 months: people who are happy to commit for a year for a lower monthly price.
• Consider O2 Plus or Ultimate: if you want rollover on a longer contract and already use O2; check the current price on O2’s own page.
• Consider Smarty 16GB at £8: if your usage is steady and you prefer a one-month plan with no annual price rises and no credit check, without rollover.

Important conditions

• Prices and plan details are as shown on iD Mobile’s pages when we checked on 9 October 2026 and can change for new customers.
• iD Mobile runs a credit check on pay monthly plans, and you need to pass it to be accepted.
• Rollover rules differ by provider and plan; add-ons and reward data are not part of iD’s rollover.
• iD’s terms say rollover is lost if you move to a new tariff.
• Network coverage varies by address, so check coverage for your home and work before you order.
• Confirm the price and data allowance in your basket before you pay.

Final decision card

Want a big allowance with leftover data carried forward? iD Mobile 60GB at £10 a month on a 30-day plan.
Happy to commit for a year? iD Mobile 50GB at £8 a month for 12 months.
Already on O2? Ask for a Plus or Ultimate plan if rollover matters to you.

FAQ

What is data rollover on a SIM only plan?

Data rollover carries unused data from one month into the next, so a quiet month can cover a busy one. The rules depend on the provider, including which data is used first and how long rolled data lasts. Check the plan page and your confirmation for the exact terms.

Does iD Mobile offer data rollover on SIM only plans?

iD says data rollover is a free feature on its plans and that unused data is added to your next month’s allowance. Its pay monthly terms say unlimited plans, add-ons and reward allowances are not eligible. Your Tariff Summary states the rollover period.

Does O2 include data rollover on SIM only deals?

O2 includes rollover on its Plus and Ultimate plans for new and upgrading customers, but not on Classic. Up to 100% of your unused main allowance can roll over, and it lasts for one billing cycle. Your main data is used before the rolled-over data.

Is rolled-over data used before or after my normal allowance?

It depends on the provider. iD says leftover data is used first, while O2 uses your main allowance first and your rolled-over data afterwards. The difference decides whether rolled data helps in a normal month or only in a heavy one.

Can I use rolled-over data abroad?

Check each provider’s roaming terms. O2 says rolled-over data can only be used in the UK. iD’s terms say any rollover counts towards the 30GB you can use while roaming in its Roam Free destinations.

Is a plan with rollover better than a bigger plan?

It depends on how much your use varies. If most months are similar, a plan sized to your average is usually the better value and rollover is a bonus. If some months are much heavier, rollover can cover them without paying for a larger allowance every month.