Caplet
A caplet is the fundamental building block of an interest rate cap — a single call option on a reference rate (such as SOFR or EURIBOR) for one specific reset period, which pays the holder the excess of the reference rate over the strike rate multiplied by the notional principal and day count fraction.
Key takeaways
- A cap is a portfolio of caplets; each caplet corresponds to one reset period of the floating rate and is priced independently.
- Caplets are priced using the Black (1976) model, treating the relevant forward rate as log-normally distributed.
- The payoff of a caplet is: Notional × max(L(T) − K, 0) × τ, where L(T) is the realized reference rate, K is the strike, and τ is the accrual period.
- Caplet implied volatilities display a 'volatility smile' or term structure, reflecting different risk premiums for different maturities.
- Stripping a cap into individual caplets allows traders to identify and trade specific maturities on the implied volatility curve.
Explanation
An interest rate cap on, say, three-month SOFR from today for two years consists of seven caplets (one for each quarterly reset period). Each caplet has its own expiration date, reference period, and can be independently priced. The ability to strip a cap into caplets is essential for market-making and risk management because it allows traders to identify which part of the volatility term structure is driving the cap's total cost.
The Black (1976) model prices a caplet as: Caplet = N × τ × P(0,T+τ) × [F × N(d1) − K × N(d2)], where N is the notional amount, τ is the accrual period (e.g., 0.25 for quarterly), P(0,T+τ) is the discount factor to the payment date, F is the forward rate for the caplet period, K is the cap strike, d1 = [ln(F/K) + σ²T/2] / (σ√T), d2 = d1 − σ√T, and σ is the caplet's implied volatility for that specific maturity. Each caplet uses the forward rate specific to its reset period, derived from the swap or futures curve.
Caplet implied volatility is extracted from market cap prices and varies across maturities, creating the caplet volatility term structure. Short-dated caplets (covering near-term periods) typically have higher implied volatility around central bank meeting dates and data releases. Longer-dated caplets incorporate macro uncertainty and the mean-reversion tendency of interest rates. The shape of the caplet vol surface — humped or monotonically declining — provides information about the market's pricing of rate volatility across time.
The SABR (Stochastic Alpha Beta Rho) model is the industry standard for interpolating and extrapolating the caplet volatility surface beyond observed market quotes. It models the forward rate and its volatility as correlated stochastic processes, capturing the empirical smile observed in caplet vols (higher vol for in- and out-of-the-money strikes than at-the-money). Traders use SABR to price non-standard strikes and generate consistent vol surfaces for Greeks calculation.
From a risk management perspective, dealers in cap books manage vega exposure by caplet maturity bucket, hedging against movements in the vol term structure using swaptions (which provide broader vol exposure) and volatility swaps. Delta hedging of caplet positions requires offsetting positions in the specific forward rate, typically using SOFR futures or interest rate swaps.
Formula
Caplet Payoff = Notional × max(L(T) − K, 0) × τ; Black Model: Caplet = N × τ × P(0,T+τ) × [F × N(d1) − K × N(d2)]
Example
A borrower has a $100 million floating-rate loan resetting every three months at SOFR. On the September 20 reset date, SOFR fixes at 5.80%. The borrower holds a cap with a strike of 5.00%. The September caplet pays: $100M × max(5.80% − 5.00%, 0) × (90/360) = $100M × 0.80% × 0.25 = $200,000. This offsets the incremental interest cost the borrower incurs above the 5.00% cap level. If SOFR had fixed below 5.00%, the caplet would expire worthless, and the borrower's interest cost would simply be SOFR plus the loan spread — just as if no cap were in place for that period.
Related terms
Alpha At The Money Bear Spread Beta Bull Spread Call Option Cap Central Bank Delta Diagonal Spread Expiration Date Futures Curve