A gilt row can carry two price changes, one for the clean price and one for the dirty price, and they differ because the accrued interest moves every day and resets on each coupon date.
Two changes on one row
On the conventional gilts list the clean price carries a change since the previous close, and the dirty price can carry one too. They are not two readings of the same figure rounded differently. The clean price measures the gilt; the dirty price measures the gilt plus the accrued interest. Because the accrued interest grows between coupons, the dirty price can move when the clean price has not moved at all, and the two changes will differ on almost every row.
Read together, the pair separates the market's move from the interest that is running. The clean change is the market's repricing; the dirty change is that repricing plus the accrual since the previous close. On a day when the market did nothing, the clean change is zero and the dirty change is the day's accrual. The size of the gap between the two therefore tells a reader how many days of interest the two reports span, and a gap that is larger than usual usually means a weekend or a bank holiday rather than a busier market.
The accrued part moves every day
Accrued interest rises with each day that passes between coupons, so the dirty price drifts upward by one day's interest even when the clean price stands still. Take an illustrative gilt with an annual coupon of 4.00, so the half-yearly payment is 2.00 per £100 nominal in a period of 182 days. One day of accrual is 2.00 divided by 182, which is about 0.011 per £100 nominal. Over a day, that is the whole difference between the clean change and the dirty change.
| Report | Clean price | Accrued interest | Dirty price | Clean change | Dirty change |
| Day one | 95.00 | 1.20 | 96.20 | — | — |
| Day two | 95.10 | 1.22 | 96.32 | +0.10 | +0.12 |
| Just before the coupon | 95.10 | 1.99 | 97.09 | 0.00 | +0.77 |
| Coupon date | 95.10 | 0.00 | 95.10 | 0.00 | −1.99 |
The table is illustrative throughout, and every input is stated: a 4.00% coupon paying 2.00 every 182 days, with the clean price held constant on the last three rows so that only the accrual moves. The point of the last two rows is that the dirty price is not a smooth series: it climbs by the accrual, then drops by roughly a whole coupon on the payment date, while the clean price is unchanged across the same dates.
Why the dirty price drops on a coupon date
The drop is not a loss. The coupon is paid to the holder on the same date, so the value that leaves the dirty price arrives as cash. A holder who watches the dirty price only sees a fall of about the coupon; a holder who watches the clean price and the coupon together sees a price that did not move and a payment received. The two views reconcile, and the reconciliation is the reason the clean price exists as the quoted figure.
A dirty price that jumps or drops by roughly a coupon is the accrued interest resetting, not the market moving. Check the clean change before reading anything into the dirty one.
Which change to read
The choice is not a matter of taste; each figure answers a different question. The clean change answers what the market did to the gilt. The dirty change answers what happened to the value of a holding on the days around the coupon, including the interest that has built up and the payment that has reset it. A valuation that needs to show the cash value of a position at a point in time wants the dirty figure; a comparison of two gilts or two dates wants the clean one.
- Use the clean change to read the market's move and to compare two gilts on the same day.
- Use the dirty price to work out the cash a purchase costs and the value of a holding on a settlement date.
- Do not compare a dirty change with a clean change: the gap between them is the accrual, not a second market move.
- Allow for the coupon reset when a dirty series drops at a payment date; the payment offsets it.
Where the series sits
The price history on a gilt's own page, opened from the list, charts the stored clean price and yield over its window, deliberately rather than the dirty price. A dirty series would show the twice-yearly resets as sharp drops and obscure the market's movement, so the history keeps to the clean figure and lets the accrued interest be read on the row. A reader reconstructing a dirty series from the history needs the accrued interest for each date, and the list is where that figure lives.
The two views are complements. The history says how the gilt has been priced; the row says what the gilt plus its running interest would cost on the report date. Reading them together is what keeps a clean price change from being mistaken for the whole change in a holding's value, and a coupon reset from being mistaken for a market fall. A reader who wants a single figure for a holding's value should take the dirty price and accept its sawtooth shape; a reader who wants a single figure for how the gilt has been priced should take the clean price and accept that it excludes the interest in transit. The sawtooth is the reason the history page charts the clean series and the row shows the accrued figure beside it.
Background, not advice
This article explains why the clean and dirty price of a gilt move differently; it is general background, not investment, tax or legal advice for any reader's circumstances. Every figure above is illustrative with its inputs stated, and the exact clean price, accrued amount and dirty price for a gilt come from the site's calculation against the day's report. The Debt Management Office and the day's market data are the primary sources for a real price or accrued figure.