Prepaid vs Postpaid Mobile Plan Cost Calculator

How this prepaid vs postpaid mobile plan calculator works

This calculator estimates and compares the total monthly cost of a prepaid mobile plan and a postpaid mobile plan based on your expected data usage. It focuses on the main recurring cost drivers you can control or estimate yourself: base plan price, included data allowance, overage charges for extra data, and any additional taxes and fees on the postpaid plan.

When you enter your typical monthly data usage in gigabytes (GB) and the details of each plan, the tool computes two numbers:

• Estimated monthly cost of your prepaid plan
• Estimated monthly cost of your postpaid plan

You can then see which option is projected to be cheaper for the usage level you selected. This does not replace reading the full terms of your plan, but it gives a clear, numerical starting point for your decision.

Formulas used in the calculator

The calculator uses simple, linear formulas for both prepaid and postpaid plans. The aim is to capture the most important cost components without reproducing every nuance of carrier billing.

Prepaid plan cost formula

For the prepaid plan, the total monthly cost is modeled as the base price plus any overage charges if you use more data than the included allowance:

• C_p = total prepaid monthly cost
• P = prepaid base monthly cost (the advertised price of the plan)
• O_p = overage cost per GB on the prepaid plan
• U = your expected monthly data usage (GB)
• D_p = data included with the prepaid plan (GB)

The max(0, U − Dp) term ensures that overage charges only apply if your usage is higher than the included data. If your usage is at or below the included data, the overage portion is zero.

Postpaid plan cost formula

For the postpaid plan, the structure is similar, but the model separately adds taxes and fees to the base plan price:

• C_s = total postpaid monthly cost
• S = postpaid base monthly cost
• F = monthly taxes and fees on the postpaid line
• O_s = overage cost per GB on the postpaid plan
• D_s = data included with the postpaid plan (GB)

As with prepaid, overage is only counted if your expected usage is higher than the included data allowance.

How to enter your numbers

The calculator fields correspond directly to the variables in the formulas.

Monthly data usage (U)

Enter how many gigabytes of data you expect to use each month across all apps and activities. If you are not sure, you can:

• Check the data usage section of your phone settings for the last 30 days.
• Look at past mobile bills, which often show your total monthly data use.
• Use a rough estimate (for example, light users might be in the 3–5 GB range, heavier streaming users often exceed 10 GB).

Prepaid plan inputs (P, D_p, O_p)

• Monthly cost ($): the advertised monthly price you pay in advance for the prepaid plan.
• Included data (GB): the amount of high-speed data included before any overage or throttling.
• Overage cost per GB ($): if your prepaid plan charges for extra data, enter the per-GB rate here. If the plan throttles speeds instead of charging extra, you can enter 0.

Postpaid plan inputs (S, D_s, O_s, F)

• Base monthly cost ($): the advertised monthly price for the postpaid line, before taxes and extra fees.
• Included data (GB): the amount of data included in the postpaid plan at full speed.
• Overage cost per GB ($): the carrier’s charge per GB when you exceed the included data limit.
• Taxes & fees per month ($): an estimate of all extra surcharges, regulatory fees, and taxes that appear on your monthly bill. You can approximate this from a recent invoice or by applying a typical percentage (for example, 10–25% of the base cost in many regions).

Interpreting the results

After you click the button to compare plans, the calculator will show the estimated monthly cost for each option and indicate which one is cheaper at your chosen usage level.

Key points when reading the results:

• If your usage is well below both included data amounts, the decision is mostly about the base prices and taxes/fees.
• If your usage is close to or slightly above a plan’s included data, small changes in your behavior can flip which plan is cheaper because of overage charges.
• If you are a very heavy user, the plan with the lowest overage cost (or with truly unlimited, non-throttled data) often becomes more economical despite a higher base price.

Use the outputs as a guide rather than a final answer. Running several scenarios (for example, 8 GB, 12 GB, and 20 GB) can help you understand your break-even points between prepaid and postpaid options.

Worked example with the default values

The default numbers in the calculator illustrate how costs change as monthly usage varies. Using those values:

• Prepaid: $40 per month, 10 GB included, $5 per GB overage.
• Postpaid: $60 base per month, 15 GB included, $10 per GB overage, plus $5 in taxes and fees.

The table below shows the resulting total monthly cost for each plan at different usage levels:

In this example, the postpaid plan has higher fixed costs but includes more data. At low and moderate usage (for example, 5–10 GB), the total postpaid cost stays flat at $65 because you remain within the 15 GB allowance. The prepaid cost also stays at $40 as long as you use 10 GB or less.

Once usage rises above the included amounts, overage charges appear. Because the postpaid overage is $10 per GB compared with $5 per GB on prepaid, the postpaid bill climbs more steeply as usage grows beyond 15 GB. At 25 GB, for instance, the example shows the prepaid plan being significantly cheaper.

You can adapt this logic to your own situation: adjust the costs and included data to match real offers you are considering, then change the usage value to see how sensitive your total monthly cost is to your actual behavior.

Prepaid vs postpaid: key cost drivers

Beyond the raw formulas, several structural differences between prepaid and postpaid plans influence your long-term costs.

The calculator focuses on the quantitative portion of this comparison: how much you are likely to pay each month based on your data usage and the plan rules. Qualitative differences, such as customer service quality or international roaming options, are important but are not modeled.

When is prepaid usually cheaper than postpaid?

Using the model above, prepaid tends to be more cost-effective when:

• Your data usage is relatively low or moderate and fits comfortably within the prepaid allowance.
• The advertised prepaid price already includes most or all taxes and fees.
• You do not need extras like device financing or premium perks.
• The postpaid plan in question has high overage charges or high recurring fees.

Postpaid can make more sense when:

• You consistently use a lot of data and the postpaid plan includes a larger high-speed allowance.
• You value bundled benefits such as device payment plans, hotspot data, or family plan discounts.
• You prefer not to monitor usage month to month and are willing to pay a bit more for convenience.

The calculator helps you see how big the cost difference is in either direction so you can decide whether the non-price benefits justify paying extra.

Assumptions and limitations

This tool deliberately simplifies mobile billing. Before relying on the numbers, keep these assumptions and limitations in mind:

• Linear overage charges: The model assumes a constant per-GB overage rate. Many carriers instead bill in blocks (for example, per extra 1 GB or 2 GB), round up partial usage, or offer temporary “top-up” packages.
• Throttling vs. overage: Some prepaid and postpaid plans throttle data speeds instead of charging for overages. In these cases, a monetary overage cost may be zero, but your experience degrades when you exceed the cap. The calculator does not quantify speed reductions.
• Promotions and discounts: Introductory pricing, auto-pay discounts, multi-line family discounts, and loyalty offers are not modeled. You can approximate them by adjusting the base monthly costs in the inputs.
• Device and installment costs: Monthly payments for phones, tablets, or other financed devices are not included. To account for them, you would need to add those charges to the relevant plan’s base cost.
• Regional tax variation: Taxes and regulatory fees can vary significantly by location and may change over time. The calculator simply uses the flat amount you enter.
• Single-line focus: The comparison is designed for a single line. Many postpaid plans become more cost-effective per line when several lines are bundled together, which is not captured here.
• Usage uncertainty: The estimates depend heavily on the usage value you choose. If your actual data use varies from month to month, you should test several scenarios to see a range of possible costs.

Because of these factors, treat the outputs as approximate guidance rather than an exact prediction of your bill. Always check your carrier’s detailed plan description and current terms before committing.

Using the calculator for better decisions

To get the most from this tool, consider running a few different cases:

• Start with your recent average data usage.
• Then test a lower-usage month and a higher-usage month.
• Compare how often each plan becomes more expensive once you factor in realistic overage behavior and taxes.

The goal is not just to find the cheapest theoretical option today, but to choose a plan that will remain comfortable for your budget if your usage or pricing changes slightly over time.