ARM Payment Shock Calculator
Stress-test your adjustable-rate mortgage by calculating the worst-case payment after the first rate adjustment, after the second adjustment, and at the lifetime rate cap.
| Rate Progression (Worst Case) | |
| Initial Rate | — |
| After 1st Adjustment | — |
| After 2nd Adjustment | — |
| Lifetime Cap Rate | — |
| Monthly Payment Progression | |
| Initial Payment | — |
| After 1st Adjustment | — |
| After 2nd Adjustment | — |
| Lifetime Cap Payment | — |
| Shock Analysis | |
| Monthly Payment Increase (Worst Case) | — |
| Annual Increase (Worst Case) | — |
An ARM payment shock calculator stress-tests an adjustable-rate mortgage by projecting the worst-case monthly payment after the first rate adjustment, the second adjustment, and at the lifetime interest-rate cap.
How ARM Caps Work — 2/2/5, 5/2/5, and Hybrid Structures
An ARM cap structure is usually written as three numbers separated by slashes: initial / periodic / lifetime. For example, a 5/1 ARM with a 2/2/5 cap can rise no more than 2% at the first adjustment, no more than 2% at any subsequent adjustment, and no more than 5% over the original rate during the loan's lifetime. The CFPB's CHARM booklet (Consumer Handbook on Adjustable-Rate Mortgages) requires lenders to disclose these caps before consummation (source: cfpb.gov).
Index, Margin, and the Fully-Indexed Rate
After the fixed period, your rate becomes index + margin, where the index is typically SOFR (Secured Overnight Financing Rate, which replaced LIBOR in 2023) plus a lender-chosen margin of 1.75-3.0%. If 30-day average SOFR is 4.30% and your margin is 2.75%, your fully-indexed rate is 7.05% — but capped by the per-period and lifetime caps in your note.
When ARM Payment Shock Becomes Dangerous
Payment shock above 25-30% of starting payment can trigger budget stress, late payments, and default risk. If your worst-case lifetime-cap payment exceeds 35% of gross monthly income, the ARM is unsuitable. The 2008 crisis featured many 2/28 and 3/27 ARMs with first adjustments doubling the payment — a structure now restricted by Dodd-Frank's qualified mortgage (QM) rules.
Last updated May 2026. Sources: cfpb.gov, CHARM booklet.
ARM Payment Shock Calculator: Worked $400K Loan Example with Current SOFR (2026)
Concrete example using June 2026 SOFR: $400,000 5/6m ARM at a 5.50% start rate with 2/2/5 caps and a 2.75% margin. Year 1–5 payment (5.50%): $2,271/month P&I. First adjustment (cap = 5.50% + 2% = 7.50%): payment recasts to ~$2,773/month on the remaining balance — a +$502/month jump, or +22% payment shock. Lifetime cap (5.50% + 5% = 10.50%): worst-case payment hits ~$3,591/month — a +$1,320/month jump, or +58% payment shock, costing $15,840/year more than the start payment. If the fully-indexed rate (current 30-day SOFR ~4.30% + 2.75% margin = 7.05%) is already above your start rate when the fixed period ends, the cap activates immediately and you take the full first-adjustment hit on day one of year 6. Stress-test your budget against the lifetime-cap row of the calculator output above, not the first-adjustment row — that is the CFPB-required worst-case scenario from your CHARM disclosure. If your projected lifetime-cap payment exceeds 35% of your gross monthly income, the loan fails the basic affordability test under Regulation Z Ability-to-Repay rules. Updated 2026-06-27.