Marcação a mercado (mark-to-market)

Marcação a mercado — mark-to-market — is the daily repricing of a bond to what it is worth if sold today. It is the most misunderstood idea in Brazilian fixed income, and understanding it prevents the most common beginner shock: "why did my safe bond go down?"

The core mechanism

A fixed-rate bond promises a set payment at maturity. Its price today is the present value of that future payment, discounted at the current market interest rate. The crucial relationship:

• When market rates rise, newly issued bonds pay more, so your older, lower-paying bond becomes less attractive — its market price falls.
• When market rates fall, your older bond looks generous, so its market price rises.

Price moves inversely to interest rates. The longer the bond’s remaining term, the more violently its price reacts to a given rate change.

A simplified illustration

Imagine a prefixed bond that will pay 1,000 in one year. If the market rate is 10%, its fair price today is:

1,000 / 1.10 = 909.09

Now suppose rates jump to 12% the next day. The same future 1,000 is now discounted harder:

1,000 / 1.12 = 892.86

The bond’s market value dropped from about 909 to about 893 — purely because rates rose, even though the promised 1,000 at maturity never changed.

The key consequence

• Hold to maturity and mark-to-market is irrelevant — you receive the agreed amount regardless of the bumps along the way.
• Sell early and you crystallise whatever the market price is that day, up or down.

Which bonds are affected

• Tesouro Selic — barely affected; built to track the rate, so it is the safe early-exit choice.
• Tesouro Prefixado and IPCA+ — fully exposed; their prices can swing meaningfully, especially long maturities.

The practical rule

Match each bond’s maturity to your goal’s timeline. If you might need the money sooner, prefer Tesouro Selic. Mark-to-market is not a defect — it is simply the price of liquidity, and it only bites if you sell before the finish line.