# ARM Mortgage Calculator (Adjustable-Rate Mortgage)

URL: https://speedycalculator.com/calculators/finance/arm-mortgage/
Last updated: 2026-10-25

> The ARM mortgage calculator estimates the payments on an adjustable-rate mortgage – 10/1, 7/1, 5/1 or 3/1 – the initial payment during the fixed period, the payment after the first rate adjustment, and the worst case at the lifetime rate cap.

## Inputs

- Mortgage balance
- Term (years)
- Interest rate at the beginning of the term (%)
- ARM type
- Expected rate change at first adjustment (%)
- First adjustment cap (%)
- Lifetime cap above the start rate (%)
- Points (% of loan) (%)
- Up-front fees

## Results

- Initial monthly payment
- Balance when the rate first adjusts
- Rate after the first adjustment
- Payment after the first adjustment
- Worst-case payment at the lifetime cap
- Total interest if the new rate stays
- Approximate APR (initial rate, with fees)

## Formula

```
Initial payment = P × i ÷ (1 − (1 + i)^−n) at the start rate
After the fixed period: balance re-amortised over the remaining term at the new rate
New rate = start rate + adjustment (limited by the first-adjustment and lifetime caps)
```

## Explanation

An ARM starts with a lower fixed rate for a few years, then resets – typically every year (or every 6 months for SOFR ARMs) – based on an index plus a margin. Caps such as 2/1/5 limit each change and the lifetime increase.

### How to use the ARM Mortgage Calculator

- **Mortgage balance**
- **Term**
- **Interest rate at the beginning of the term**
- **ARM type**
- **Expected rate change at first adjustment**
- **First adjustment cap**
- **Lifetime cap above the start rate**
- **Points (% of loan)**
- **Up-front fees**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Enter the loan amount, term, start rate, ARM type, expected first adjustment, the first-adjustment cap and the lifetime cap.

### Tips

- Get a written loan estimate and compare the APR, not just the interest rate.
- Include property tax, insurance, HOA fees and maintenance in your monthly budget.
- A larger down payment lowers the payment and can remove mortgage insurance.
- Rates change daily – use a current quote for the most accurate result.

### Common mistakes to avoid

- Looking only at the monthly payment instead of the total cost of the loan.
- Forgetting closing costs and fees.
- Assuming home prices always rise.

### Inputs explained

FieldWhat to enterMortgage balanceAmount in your currencyTermIn yearsInterest rate at the beginning of the termIn %ARM typeChoose one: 10/1 ARM, 7/1 ARM, 5/1 ARM, 3/1 ARMExpected rate change at first adjustmentIn %First adjustment capIn %Lifetime cap above the start rateIn %Points (% of loan)In %Up-front feesAmount in your currency

### What each result means

- **Initial monthly payment** = pmt([Interest rate at the beginning of the term]÷1200, [Term]×12, [Mortgage balance])
- **Balance when the rate first adjusts** = bal([Mortgage balance], [Interest rate at the beginning of the term]÷1200, [Term]×12, [ARM type]×12)
- **Rate after the first adjustment** = [Interest rate at the beginning of the term] + min([Expected rate change at first adjustment], [First adjustment cap])
- **Payment after the first adjustment** = pmt([Rate after the first adjustment]÷1200, ([Term] - [ARM type])×12, [Balance when the rate first adjusts])
- **Worst-case payment at the lifetime cap** = pmt(([Interest rate at the beginning of the term] + [Lifetime cap above the start rate])÷1200, ([Term] - [ARM type])×12, [Balance when the rate first adjusts])
- **Total interest if the new rate stays** = [Initial monthly payment]×[ARM type]×12 + [Payment after the first adjustment]×([Term] - [ARM type])×12 - [Mortgage balance]
- **Approximate APR (initial rate, with fees)** = rate([Term]×12, 0 - [Initial monthly payment], [Mortgage balance] - [Up-front fees] - [Mortgage balance]×[Points (% of loan)]÷100)×1200

### Example values

How the main result changes when **Mortgage balance** changes and everything else stays at the example values above:

Mortgage balancePayment after the first adjustment200,0001,375.33300,0002,062.99400,0002,750.65500,0003,438.32600,0004,125.98800,0005,501.31

## Worked example

Inputs — Mortgage balance: $400,000; Term: 30 years; Interest rate at the beginning of the term: 6 %; ARM type: 5/1 ARM; Expected rate change at first adjustment: 1.5 %; First adjustment cap: 2 %; Lifetime cap above the start rate: 5 %; Points (% of loan): 0 %; Up-front fees: $3,000.

Results — Initial monthly payment = $2,398.20; Balance when the rate first adjusts = $372,217.43; Rate after the first adjustment = 7.5 %; **Payment after the first adjustment = $2,750.65**; Worst-case payment at the lifetime cap = $3,648.15; Total interest if the new rate stays = $569,088.31; Approximate APR (initial rate, with fees) = 6.07 %.

## FAQ

**What does 5/1 ARM mean?**
The rate is fixed for 5 years, then adjusts every year.

**What are ARM caps?**
Limits on the first adjustment, each later adjustment and the total increase.

**Is an ARM a good idea?**
It can be if you expect to sell or refinance before the fixed period ends.

**What formula does the ARM Mortgage Calculator use?**
Initial payment = P × i ÷ (1 − (1 + i)^−n) at the start rate.


## Sources

- [Consumer Financial Protection Bureau](https://www.consumerfinance.gov/)