28-Day vs. 30-Day Mobile Plans: What You Really Pay

A 28-day mobile plan gives you your included data, calls and other services for 28 days before the plan expires or renews. Put simply, that’s four weeks, not a calendar month.

It’s an easy detail to overlook. When we hear “monthly”, we naturally think we’re paying once a month. But a 28-day prepaid mobile plan follows a fixed four-week cycle instead. This means your renewal date can shift through the calendar each time your plan renews.

The calculation is simple: 28 × 13 = 364 days. So, there are 13 complete 28-day periods in 364 days.

That’s useful to know when comparing a 28-day recharge with a 30-day prepaid mobile plan. It doesn’t mean a 28-day plan is automatically bad value. It simply means you need to compare plans over the same timeframe to see what you’re really paying.

Let’s look at two mobile plans: Plan A costs $25 every 28 days, while Plan B costs $25 every 30 days. At first, they look like the same deal. You’re paying $25 for both, but that $25 covers a different number of service days.

With Plan A, you’re paying about $0.89 per day ($25 ÷ 28). Plan B works out to about $0.83 per day ($25 ÷ 30). If everything else is the same, the 30-day plan gives you two more days of service for the same price.

So, the price you see isn’t always the full picture.

Of course, mobile plans aren’t always that simple. One plan might come with more data, better coverage or features that suit you better. That’s why it’s worth looking at the recharge period alongside everything else.

The ACCC says businesses should make their pricing clear and accurate and avoid giving customers a misleading impression. Its 2026–27 priorities also include misleading pricing and claims in essential services, including telecommunications.

This doesn’t mean every 28-day plan is poor value or misleading. It’s simply worth checking exactly what you’re paying for before you sign up.

You don't need to be a maths expert to work out the true cost of mobile plans. 

A few simple checks can make the comparison much easier.

1. Check the Mobile Plan Renewal Period

First, find out exactly how long your recharge lasts. It could be 28 days, 30 days, a calendar month, 7 days, 90 days or even 365 days.

Don't assume that “monthly” means the same thing across providers. Check the plan's stated validity period before comparing it with another plan.

For example, Yomojo prepaid mobile plans currently renew every 30 days. Check plan validity for more information.

2. Calculate the Cost Per Day

This is one of the easiest ways to compare plans with different recharge periods. Just divide the plan price by the number of days to see how much you’re paying per day.

For our $25 example, a 28-day plan costs about $0.89 per day ($25 ÷ 28), while a 30-day plan costs about $0.83 per day ($25 ÷ 30).

This gives you a clearer way to compare plans and see what you’re really getting for your money.

3. Compare Both Plans Over the Same Period

Want to make the comparison even clearer? Convert both plans to the same timeframe.

For an estimated 30-day cost, use:

Price ÷ validity days × 30

For an annualised comparison, use:

Price ÷ validity days × 365

Annualised figures are estimates based on continuous service. They’re useful for comparing plans, but they won’t necessarily match the exact dates or number of recharges you make in a particular year.

And remember, this isn’t about declaring a winner. A 28-day plan isn’t automatically worse value. The calculation simply shows that two plans with the same advertised price aren’t necessarily equivalent when they cover different numbers of days.

If you’re new to comparing prepaid plans, what to know before choosing a prepaid plan can also help you work out which features matter most to you.

4. Check What You'll Pay After the Promotion

Here’s another thing that’s easy to miss.

You see “$1 for your first month!” and think, “Sounds great.” But what happens after that?

When comparing a promotional prepaid mobile plan, look at three things: the promotional price, how long the offer lasts, and the ongoing price.

A discount might only apply to your first recharge, your first few renewals or a set promotional period. The important question is: What will I actually pay once the promotion ends?

The ACCC says businesses should make sure advertising claims are truthful, accurate and based on reasonable grounds, and that they can support the claims they make.

Yomojo CEO James Linton has more than 20 years of experience in Australian telecommunications. In a recent LinkedIn post, he highlighted something that’s easy to overlook when comparing mobile plans: how long the recharge lasts.

His message is simple: don’t just look at the price. Ask three questions: How much am I paying? How many days do I get? And what will I pay when the promotion ends?

As James puts it, “Twenty-eight days is four weeks, it is not a month.”

You can read James Linton’s full LinkedIn post.

These questions aren’t just useful when comparing Yomojo. They can help you compare almost any 28-day prepaid mobile plan, 30-day plan or other prepaid recharge.

Now let’s put that approach into practice.

Yomojo’s current Starter promotion is a good example. It combines an introductory price with a 30-day renewal period, so you can see how the offer works beyond the headline price.

The current offer includes:

• $1 for the first 30 days for eligible customers bringing their existing number

• 50% off for Months 2–6

• 29GB of data

• 30-day renewals

• Standard pricing from Month 7

The current offer is listed as ending 30 September 2026. Activation is required within 45 days, and auto-renewal is required.

Always check the latest offer before signing up, as promotional prices and terms can change.

The standard Starter price is currently $24.90 every 30 days.

During the promotion, you’ll pay $1 for the first 30 days. For Months 2–6, you’ll get 50% off the standard price, bringing each renewal down to $12.45.

So, over the first six promotional periods:

$1 + ($12.45 × 5) = $63.25

That means you’ll pay $63.25 for your first six 30-day periods.

How Many Days Does That $63.25 Cover?

Yomojo’s Starter Plan renews every 30 days, so six renewal periods give you:

6 × 30 = 180 days

For comparison, six 28-day periods would give you:

6 × 28 = 168 days

That’s a difference of 12 days.

So, over six renewal periods, a 30-day plan gives you 12 more days of service than a 28-day plan.

What Happens After the Promotion?

From Month 7, the standard Starter price applies, which is currently $24.90 every 30 days.

That’s why it’s worth looking beyond the introductory offer. A plan might seem like a great deal while the promotion is running, but the cost can look quite different once the discount ends.

Before signing up, make sure you know what you’ll pay during the promotion and what you’ll pay afterwards.

The CIS is worth checking before you sign up. It gives you important information about a telecommunications product, including its features, pricing, inclusions, exclusions, fees and charges. You can read more in ACMA’s guidance on choosing the right product or plan.

If you’re switching providers, you can also learn more about how to keep your number when switching mobile plans.

Before choosing your next prepaid plan, ask: