The Real Cost of Buying Dollars: Spreads, Fees and How to Calculate Your Effective USD/KRW Rate

The rate you see on the news is not the rate you get. Learn how bank spreads, "preferential rate" discounts, card FX fees and transfer charges combine into one effective exchange rate — with formulas and a Korea-focused worked example.

USD/KRW closes at 1 380 on the news. You walk into a bank and pay 1 404 per dollar. You send money abroad and somehow the recipient gets even less. None of this is a scam; it is a stack of small percentages that nobody shows you as one number. This article shows you how to compute that one number — the effective rate — for any transaction.

Three rates you need to tell apart

The gap between buy and sell is the spread. For USD cash at major Korean banks it is about 1.75% each way; for transfers about 1%. Minor currencies are far worse — 4–12% for cash in currencies the bank doesn't hold much of.

"90% preferential rate" — what it actually means

Korean banks and apps advertise 환율 우대 90%. That does not mean a 90% discount on the dollar. It means they refund 90% of the spread:

Effective rate = mid + spread × (1 − preference)
               = 1 380 + 24.15 × (1 − 0.90)
               = 1 382.4       (vs 1 404.2 with no preference)

So 90% preference on a 1.75% cash spread leaves you paying 0.175% above mid. 100% preference is truly mid-market (some fintech apps offer this on weekdays). 50% preference — common for walk-in customers without an app — still costs 0.875%.

Formula: effective rate including flat fees

Whenever there is a fixed fee (a wire charge, a card foreign-transaction surcharge, an ATM fee), fold it into the rate:

Effective rate = (total KRW paid) / (USD received)

Worked example — sending $3 000 to a US brokerage:

Bank TT selling rate with 70% preference:
   1 380 + 13.39 × 0.30 = 1 384.0 → 3 000 × 1 384.0 = ₩4 152 000
Outgoing wire fee (bank)          + ₩15 000
Intermediary bank fee (~$20)      + ₩27 600 (deducted from the dollars: you get $2 980)
Effective rate = (4 152 000 + 15 000) / 2 980 = ₩1 398.3 per dollar

Headline: "70% preferential rate". Reality: 1.3% above mid — worse than the no-fee app quoting 1 384 flat. Flat fees are brutal on small transfers: the same ₩15 000 + $20 on a $300 transfer pushes the effective rate to ₩1 530, an 11% premium.

Credit cards abroad: three layers

Paying with a Korean card in the US typically stacks:

1. Card network (Visa/Mastercard) conversion at their own rate — usually within 0.1–0.3% of mid.
2. Network fee ≈ 1.0–1.1%.
3. Issuing bank foreign-use fee ≈ 0.2–0.5% (sometimes waived on travel cards).

Total ≈ 1.3–1.9% — comparable to buying cash at 0% preference, but with no ATM fee and no cash to lose. Always decline "pay in KRW" at the terminal (Dynamic Currency Conversion): the merchant's processor applies its own rate, typically 3–7% worse, on top of everything above.

Dollar accounts and timing

Investors who buy USD to hold — to buy US stocks, or simply as a hedge against KRW — face a different question: not which counter but when. Two useful calculations:

Break-even move. If your round-trip cost (buy + sell spread and fees) is 0.6%, the exchange rate must move more than 0.6% in your favour before you profit. At 1 380 that is about ₩8. Anything less is noise you pay for.

Averaging. If you exchange a fixed KRW amount every month, your average cost is the harmonic mean of the rates, which is always at or below the arithmetic mean — you automatically buy more dollars when they are cheap. Example over three months at 1 300, 1 400 and 1 450 with ₩1.4 M each:

Dollars bought = 1 400 000/1 300 + 1 400 000/1 400 + 1 400 000/1 450
              = 1 076.9 + 1 000.0 + 965.5 = 3 042.4
Average cost  = 4 200 000 / 3 042.4 = ₩1 380.5   (arithmetic mean of rates: 1 383.3)

Quick decision table