YTM: yield to maturity from your trade price
YTM (Yield to Maturity) is a bond's effective annual yield if you hold it to maturity and reinvest the coupons. Firewire calculates YTM not from an abstract market quote, but from your real trade price.
What YTM is
A bond's coupon rate is fixed, but your yield depends on the price at which you bought it. Buy below par — your yield is higher than the coupon; buy above par — it's lower. YTM ties it all together: it's the discount rate at which the present value of all future payments (coupons + redemption) equals the price you paid.
Formula
YTM is the rate y that equates the "dirty" purchase price (price + accrued coupon income) with the discounted stream of payments:
The equation is solved numerically (just like XIRR). For amortizing bonds the par value is redeemed in parts, so instead of a single payment N the stream contains several partial redemptions.
Why "from your trade price" matters
Exchange terminals show YTM from the current market price — it changes every second and has nothing to do with what you personally locked in. Firewire calculates YTM from your actual purchase price, so the number reflects the real locked-in yield of your specific position.
Example (intuition)
A bond with a par value of 1000 ₽, a 10% annual coupon, 3 years to maturity. If you buy it for 950 ₽ (below par), you receive not only the coupons but also +50 ₽ of difference at redemption — so the YTM will be higher than the 10% coupon. If you buy it for 1030 ₽, the YTM will be lower than 10%. The exact value is found by solving the equation above.
How Firewire calculates it
For each bond position Firewire takes the trade price, adds the accrued coupon income paid, subtracts commissions and expected taxes, builds the schedule of future coupons and redemptions (including amortization) and numerically finds the YTM. Alongside it, the duration is shown — so you see not only the yield but also the interest-rate risk of the position.