ARM vs Fixed Rate Calculator

The difference is what happens to your interest rate over time.

A fixed-rate mortgage locks your interest rate permanently. The principal and interest portion of your payment is identical every month for the full loan term — 15, 20, or 30 years. Your payment in year 1 equals your payment in year 30. This predictability is the core value proposition.

An adjustable-rate mortgage (ARM) starts with a fixed rate for an initial period (3, 5, 7, or 10 years), then adjusts periodically based on a market index. A 5/1 ARM is fixed for 5 years, then adjusts every year. During the fixed period, ARMs carry a lower rate than comparable fixed products — the tradeoff for accepting rate risk later.

Key numbers to compare:

• Current 30-year fixed rate vs. 5/1 ARM rate — the spread tells you your initial savings
• ARM cap structure (e.g., 2/2/5) — tells you your maximum possible rate
• Your planned time in the home vs. the ARM's fixed period

An ARM makes financial sense in four specific situations:

1. You're selling or refinancing before the fixed period ends

• If you have a 7/1 ARM and plan to sell in 5 years, you never face an adjustment
• You capture 5+ years of lower payments with zero rate risk
• The median American homeowner moves every 8–13 years — many never reach their ARM's adjustment date

2. You believe interest rates will fall

• After your fixed period, your ARM adjusts downward automatically if the index falls
• Fixed-rate borrowers must pay $3,000–8,000 in closing costs to refinance to a lower rate
• ARM borrowers get the rate reduction for free

3. The rate difference is material (1%+ typically)

• At a $400,000 loan: 1% rate difference = ~$250/month savings, $15,000 over 5 years
• Below 0.5% spread: the complexity of an ARM rarely justifies the risk

4. You need a lower payment to qualify

• A lower initial ARM rate reduces your qualifying monthly payment and DTI ratio
• Can increase your purchase power significantly — sometimes by $30,000–$50,000 in home price

ARM names follow a simple X/Y format: initial fixed period / adjustment frequency.

You may also see ARMs labeled with 6-month adjustments (5/6, 7/6), meaning the rate adjusts every 6 months after the initial period, not annually. These have become more common since the transition from LIBOR to SOFR. More frequent adjustments mean your rate responds faster to market changes — both up and down.

Rule of thumb: match your ARM's fixed period to your expected stay. If you'll move in 7 years, a 7/1 ARM gives you the full fixed period with zero adjustment risk.

Rate caps are the guardrails that prevent your ARM from adjusting to an unaffordable level.

Most ARMs use a three-part cap structure, shown as three numbers (e.g., 2/2/5):

• First number — Initial Cap: Maximum increase at the very first adjustment. If your ARM starts at 5.5% with a 2% initial cap, the first adjustment can go no higher than 7.5%.
• Second number — Periodic Cap: Maximum increase at each subsequent annual adjustment. With a 2% periodic cap, the rate can only rise 2% per year after the initial adjustment.
• Third number — Lifetime Cap: The maximum the rate can ever rise above the starting rate. A 5% lifetime cap on a 5.5% starting rate means your rate can never exceed 10.5%.

Example — 5/1 ARM at 5.5% with 2/2/5 caps:

Caps protect you from catastrophic scenarios. Use this calculator's worst-case scenario button to see what your payment looks like if all caps are triggered — then decide if that payment is still manageable.

New ARM Rate = Index + Margin, subject to your caps.

The Index: A market benchmark rate that changes with economic conditions. After the 2023 transition away from LIBOR, most new ARMs use SOFR (Secured Overnight Financing Rate) or the 1-Year Constant Maturity Treasury (CMT). Your loan documents will specify which index applies to your loan.

The Margin: A fixed percentage your lender adds to the index, set permanently at origination. Typical margins range from 2.5% to 3.5%. If your margin is 2.75% and the SOFR is 4.0%, your rate adjusts to 6.75% — subject to your caps.

Your loan's "fully indexed rate" (index + margin at origination) is an important number to note — this is roughly what your rate would be if it adjusted today. If the fully indexed rate is already above your fixed rate, you know exactly which direction your first adjustment will likely go.

Your loan disclosure (ARM Disclosure Form) must disclose the index, margin, cap structure, and a worst-case payment scenario by federal law — review it carefully before signing.

Your payment increases, but rate caps prevent the worst-case scenarios people fear.

Let's walk through a real scenario: You have a 5/1 ARM at 5.5% with 2/2/5 caps on a $350,000 loan.

Your monthly P&I during the fixed period: ~$1,987/month

Worst-case scenario (all caps triggered, year by year):

Even in the absolute worst case, the payment doesn't become unlimited — the cap structure controls the maximum. The 30-year fixed rate equivalent was likely around 7.0–7.5% when you took the ARM, so the worst-case ARM payment converges with what you'd have paid on a fixed rate anyway.

The actionable takeaway: Run this calculator's worst-case scenario and ask: "Is this payment still manageable in my budget?" If yes, the ARM may still make sense. If no, the fixed rate's certainty is worth the premium.

Counterintuitively, ARMs can be more attractive when rates are high — not less.

Here's the logic:

• When rates are high, there's more room for them to fall than rise further
• An ARM that adjusts downward as rates normalize saves you money without refinancing
• The rate spread between ARMs and fixed loans is often wider during high-rate environments, meaning larger initial savings
• If you refinance to a fixed rate when rates eventually fall, you benefit regardless

The historical pattern: During the 2022–2024 high-rate environment, many buyers who chose ARMs benefited as rates began to moderate. Those who chose fixed rates locked in at the peak — not because ARMs are always better, but because rate timing matters.

The risk: Rates can stay high longer than expected, or rise further before falling. If you need certainty and your budget can't absorb a worst-case adjustment, a fixed rate is always the safer choice. Use this calculator's worst-case scenario to verify your ARM is affordable even if rates rise to the cap maximum.

The break-even point is the month when the fixed rate's cumulative cost drops below the ARM's cumulative cost.

The logic:

• During the ARM's fixed period: you're accumulating savings (ARM payment < fixed payment)
• After the ARM adjusts: if the ARM rate is higher, you start "spending" those saved dollars
• The break-even is when the cumulative costs cross — ARM ahead before it, fixed ahead after it

If you sell or refinance before the break-even month → ARM wins

If you keep the loan past the break-even month → Fixed rate wins (under that rate scenario)

This calculator tracks cumulative costs month by month, including the rate adjustment impact, and shows you the exact break-even month. It recalculates for each of the four rate scenarios (your estimate, best case, moderate rise, and worst case) so you can see how sensitive the break-even is to rate assumptions.

Important nuance: The break-even assumes you keep both loans with no extra payments and no refinancing. In practice, most ARM holders refinance before or at the adjustment date — which restarts the analysis entirely.