When Convexity Adjustments Apply to Forward Rates and Futures

# Convexity adjustment for a forward swap rate


# Convexity adjustment for a forward swap rate












I recently heard that for a forward swap rate (for example, the fixed rate of a swap that will start in one year and end in five years), I need to do a convexity adjustment in order to get the right number. Is it true, and if so why?

## Answer by imachabeli (score 10)

https://quant.stackexchange.com/a/7833

First of all it is not clear what exactly you mean by right number, you definitely do not adjust forward swap rate.

You probably mean adjusting euro dollar futures contract rates so that you can later use these values to fit the swap/forward libor curve.

Reason for adjustment is simple. If you are short ED futures and rates go higher futures price drops and you make money. Clearing house of exchange reimburses you excess margin and you can reinvest them at higher rate. If rates go lower you have to put extra cash into your margin account, you can borrow this money at lower rate.

Assuming some distribution and forward rate model(and here things vary from simple vasicek model to extremely complicated example Peterbarg) one can compute how much this advantage is in dollars and convert it to basis points.

Pure intuition tells that longer is the expiry higher is the adjustment also higher is the vol higher is the adjustment.

So the end result is that being short gives you advantage and market participants are aware of it and penalize short side by that amount.

## Answer by Christian Fries (score 3)

https://quant.stackexchange.com/a/7850

Given an index $t \mapsto S(t)$ (this may be a forward swap rate) and some value process $t \mapsto A(t)$ (this may be a swap annuity) we assume that $S/A$ is a traded product (which is true if $S$ is the forward swap rate and A is the corresponding (!) swap annuity. Then the future payoff $S(T) \cdot A(T)$ can be values as $S(t) \cdot A(t)$ (since $S$ is a martingale under the measure $Q^A$).

Now, if we consider the payoff $S(T) \cdot P(T)$ (say for example if $P$ is the zero coupon bond with maturity $T$) then the value can be expressed as $S'(t) \cdot P(t)$ where $S'(t)$ is the so called convexity adjusted rate, that is $S'(t) = E(S(T) \cdot \frac{P(T)/P(t)}{A(T)/A(t)})$ (with expectation under $Q^A$). The convexity adjustment is a correlation term coming from the correlation of the index to the change of the payment.

That said: If you need a convexity adjustment depends on how the index is paid.

## Answer by Alexey (score 3)

https://quant.stackexchange.com/a/7863

Most likely the question is about CMS rate convexity adjustment. i.e. today value of a swap rate that fixes at some future time T.

Mathematically, the adjustment arises from different measures (annuity versus forward measure).

This is a good reference http://www.math.nyu.edu/~alberts/spring07/Lecture4.pdf

As a rule of thumb, the size of the adjustment depends on

- CMS maturity

- Level of vol

- skew

For the latter the classical reference is http://www.gorillasci.com/documents/convexity.pdf

## Answer by Matt Wolf (score 1)

https://quant.stackexchange.com/a/7813

Yes, an adjustment has to be made and the reason is that a forward curve now will evolve and not be the same as the future spot curve. For example, a one year forward today is not equal to your spot rate a year hence. So spot curve discount factors have to be adjusted or directly replaced through the forward DFs.

Convexity adjustments are already supposed to be made during the forward libor curve construction.

## Answer by Phil H (score 0)

https://quant.stackexchange.com/a/7840

For a vanilla forward-start swap, I would agree with imachabeli; convexity is an adjustment for the non-linearity of the quoted fixed rate dependence on the floating note. If expected Libors rise 1bp, the fixed leg can be increased 1bp to compensate.

Convexity adjustments are made as standard to interest rate futures (i.e. 3m); with the next futures date (Jun13) 2 months away, the front contract convexity adjustment is less than .1bp, so it makes no real difference. By contrast at 5y (Jun18), the 21st contract convexity is around 15bp.

It is possible the question is about swap futures, which deal over the futures dates, and which are therefore forward starting. As these are also futures, and deliver margin payments, there is a convexity adjustment to be made as per 3m futures.