COFIX Explained: Why One Index Reads 3.05% and 2.54% on the Same Day

Published on the same day, COFIX reads 3.05% under one basis and 2.54% under another — a gap of 0.51 percentage points. The Korea Federation of Banks consumer portal shows the 15 July 2026 release at 3.05% for the new-lending basis, 2.94% for the outstanding-balance basis and 2.54% for the expanded balance basis, with a short-term COFIX of 2.90% published on 29 July. Four live numbers at once. Which one is written into your mortgage contract decides the base rate you pay at any given moment.

Where the four versions diverge

COFIX (Cost of Funds Index) aggregates the funding rates of eight major Korean banks. The split is about which funds get counted. The new-lending basis looks only at money raised during that month; the outstanding-balance basis looks at the entire funding balance at month end. The expanded balance basis widens the pool further to include other deposits, borrowings and settlement-type funds — and because that pool contains cheap, near-zero-cost money, the index level itself comes out lower (Toss Bank).

The speed difference flips sign with direction. In a rising-rate phase the slow-moving expanded basis favours the borrower; in a falling-rate phase the cuts arrive late for exactly the same reason. COFIX has now risen three months running in 2026 — 0.08pp in April, 0.01pp in May and 0.15pp in June.

What 0.51 percentage points costs in interest

Comparing index levels alone is abstract, so the table below adds the same 1.00pp spread and computes first-year interest on a 300 million won loan, before any principal repayment.

Stopping there produces the wrong decision. Banks routinely attach a wider spread to products built on a lower index to recover the same funding cost. If the expanded-basis product carries a spread 0.5pp higher, the 1.53 million won gap above effectively disappears. The comparison has to bundle index plus spread, together with the reset cycle.

For a floating-rate borrower the dates that matter are not central bank meetings, but the 15th of each month and the reset date written into the loan.

On reset day, everything accumulated arrives at once

Reset cycles are typically six or twelve months. The index is published monthly, but your rate changes only on the reset date, absorbing the whole accumulated move in one step. April through June alone add up to 0.24pp — about 720,000 won more interest per year on 300 million won. New borrowers feel it sooner: the day after the June release, KB Kookmin Bank's six-month floating mortgage range moved from 4.02-5.42% to 4.17-5.57% and Woori Bank's from 4.39-5.59% to 4.54-5.74%, with jeonse (lump-sum deposit lease) loans shifting from 3.66-5.06% to 3.81-5.21%.

If you are weighing a switch to a fixed rate, the relevant benchmark is not today's floating rate but the COFIX that will apply on your reset date. Two more releases may still land before then.

What to check

• The index name in your loan agreement — new lending, outstanding balance, expanded balance or short-term (it varies by product even within one bank)
• Your reset cycle and next reset date, and how many monthly releases fall in between
• The spread and any preferential-rate items, and whether those conditions still hold at reset
• The publication calendar on the Korea Federation of Banks portal — the 15th monthly, weekly for short-term COFIX
• If refinancing, the remaining early-repayment penalty period plus stamp duty and registration costs

Sources

• COFIX disclosures (Korea Federation of Banks consumer portal)
• June COFIX tops 3% for the first time in 17 months; loan rates rise again (Digital Times)
• COFIX rises for a third month to 3.05% on the new-lending basis (Good Morning Economy)
• What is COFIX, and how it relates to floating mortgage rates (Toss Bank)