Interest & Mortgage
Deposit · savings · mortgage + after-tax.
Interest & Mortgage · Blog
Articles about Interest & Mortgage
US savings, CDs, and mortgage math. APR vs APY, bi-weekly payments, IRS interest rules.
For US readers, interest math has a few specific quirks: APR vs APY (the difference between simple annual rate and effective compounded rate), bi-weekly mortgage payments cutting 4–6 years off a 30-year loan, the 1099-INT $10 reporting threshold, and the Federal Reserve's effective fed funds rate as the upstream signal. These guides walk through each in plain English with the formula visible.
What this category covers
- APR vs APY — when does it matter?
- Bi-weekly mortgage payments — 4 to 6 years saved
- 1099-INT — IRS reporting and the $10 threshold
- Compound frequency — daily vs monthly vs yearly
Primary sources
- Federal Reserve — effective fed funds rate
- IRS — Topic 403 (interest received) and Form 1099-INT
- Consumer Financial Protection Bureau — mortgage guides
Want to use the tool directly? Head to Interest & Mortgage. The articles below are sorted newest first.
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Foreign Currency Deposits in 2026: Hedging USD Exposure for Non-US Savers
Top US 6-month CDs sit around 4.0-4.2% APY in May 2026, edging out most foreign currency deposits after FX volatility.
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U.S. Retirement Stack 2026: Social Security, 401k, Roth IRA, HSA
Build a $1M retirement nest egg in 30 years. Social Security baseline + 401k max + Roth IRA + HSA stacking strategy. Why HSA is the best-kept retirement secret.
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ARM vs Fixed Mortgage: Lifetime Cap Math and the 5/1 Decision (2026)
5/1 ARM at 6.10% vs 30-year Fixed at 6.85% on a $400K loan. Lifetime cap caps reset at +5%p. When ARM wins, when Fixed protects, and the 5-year sale strategy.
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529, Roth IRA for Kids, UTMA: U.S. Children's Savings Strategies (2026)
529 plans grow tax-free for education. Roth IRA for Kids works if your child has earned income. UTMA gives flexibility but loses tax benefits. The right account for your child's future.
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Refinance Break-Even: When to Pull the Trigger on a New Mortgage
A 0.5% rate drop with $5,000 in closing costs breaks even at month 23 on a $400K loan. The math, the rule of thumb, and when refinancing is wrong even if the rate is lower.
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401(k) Lump Sum vs Rollover IRA: The Decision at Separation (2026)
When you leave the job, what happens to your 401(k)? Lump sum vs Rollover IRA, the 60-day rule, the 10% early withdrawal penalty, and SECURE 2.0 RMD changes explained.
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Nominal vs Real Interest: What Your 5% HYSA Actually Earns After Inflation
5% HYSA APY minus 33% federal+state tax = 3.34% net. Subtract 2.5% inflation = 0.84% real. The math behind why your savings might be losing purchasing power even at 'high' rates.
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Personal Loan vs HELOC vs Credit Card Line — $10K for 12 Months in 2026
Borrowing $10,000 for 12 months looks simple until you compare a 10% personal loan, a 9% HELOC, and a 22% credit card line. Real cost, unused-line pricing, revolving vs installment, and DTI impact.
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Roth IRA, 401k, and HSA: U.S. Tax-Advantaged Accounts in 2026
Max contributions for 2026: Roth IRA $7,000, 401k $23,500, HSA $4,300. The triple-tax-advantaged accounts that can save high earners $10K+ per year. Priority order and limits explained.
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Loan vs Lease vs Cash — 5-Year Total Cost on a $30K Car (2026)
Finance a $30K car at 7.5% APR for 60 months and you pay $36,180 — $6,180 in interest. Lease 36 months for $15,120 with no equity. Cash buyers forgo $8,300 in compound returns. Here's the math.
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$100K Over 10 Years: Savings vs Stocks Math (Why the Gap Is $130K)
Park $100K in HYSA at 5% for 10 years and you have ~$163K. Invest the same $100K in S&P 500 at 10% historical average — about $260K. The math behind the $97K gap and why time horizon matters.
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Where to Park Your Emergency Fund in 2026 (HYSA, T-Bills, or Money Market)
A 6-month emergency fund of $30,000 sitting at 0.01% interest loses $1,500 a year vs HYSA. The right home for emergency cash: HYSA, T-Bills, or money market funds — and when to use each.
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Korea's LTV Cap: 70% vs US 80% vs Japan 100% (2026)
Korea's 2026 mortgage LTV is 70% in non-regulated zones, 50% in regulated zones, with a +10pp bonus for newlyweds and first-time buyers.
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30Y vs 15Y Mortgage: When the Shorter Loan Actually Wins
On a $400K mortgage, a 15-year fixed costs ~$750 more per month than a 30-year and saves ~$335K in lifetime interest. The real decision is not the rate — it's the cash-flow risk.
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Your $100K HYSA Math, After Federal and State Tax (2026)
A 5% APY HYSA headline looks great until your federal bracket and state tax pull the real number down. Here is the after-tax math for $100K parked one year — TX vs CA, HYSA vs T-bill.
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Korea's DSR 40%: How a Government Cap Beats US DTI 43%
Korea legally caps household debt service at 40% of income. A ₩70M household can borrow about ₩460M (~$345K) for a mortgage — but a single auto loan can chop that to ₩140M.
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APR vs APY: How $100K at 4% Splits Into $324K, $331K, or $332K Over 30 Years
$100,000 at 4% nominal for 30 years pays $324,340 with annual compounding, $331,350 monthly, $331,990 daily.
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12-Month Savings Ladder vs Lump-Sum CD — Same APY, Half the Interest (2026)
A 4.5% one-year CD on $100,000 earns roughly $4,500. The same $100,000 dollar-cost averaged monthly into a 4.5% APY savings account earns about $2,440.
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Equal Payment vs Declining Principal: $89K of Interest Most Buyers Never See
On a $300K · 6.5% · 30-year mortgage, the standard equal-payment loan costs about $89,000 more in lifetime interest than a declining-principal schedule — if your lender even offers one.
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Simple vs Compound Interest: The 30-Year Gap That Costs You $111,000
Same 4%, same $100,000. After five years the gap is barely $2,000. After thirty years it is $111,000. Here is why and what to do about it.