Price a fixed-rate bond with a REST API (clean/dirty price, YTM, duration)

01The call

A 5% annual bond issued 2024-07-01, maturing 2029-07-01, settling 2026-10-15, priced at a flat 4% yield:

curl -s https://quantbox.dev/v1/bonds/price \
  -H "X-API-Key: $KEY" -H "Content-Type: application/json" \
  -d '{"coupon_rate": 0.05,
       "issue_date": "2024-07-01",
       "maturity_date": "2029-07-01",
       "settlement_date": "2026-10-15",
       "frequency": "annual",
       "day_count": "act/act",
       "yield_rate": 0.04}'

Runs on the free tier — 500 calls/month, no credit card. Get a key.

{
  "clean_price": 102.4892,
  "dirty_price": 103.9413,
  "accrued": 1.4521,
  "ytm": 0.04,
  "modified_duration": 2.4748,
  "convexity": 8.7012,
  "bpv": -0.0257,
  "conventions": {
    "prices": "per 100 of face value (market quote convention)",
    "settlement": "as provided (settlementDays=0, no implicit T+2)",
    "ytm": "compounded annual, act/act",
    "zero_curve": "zero rates continuously compounded, act/365",
    "calendar": "TARGET, unadjusted schedule"
  }
}

02Same call in Python

import requests

KEY = "YOUR_API_KEY"  # free key from https://quantbox.dev

r = requests.post("https://quantbox.dev/v1/bonds/price",
    headers={"X-API-Key": KEY},
    json={"coupon_rate": 0.05,
          "issue_date": "2024-07-01",
          "maturity_date": "2029-07-01",
          "settlement_date": "2026-10-15",
          "frequency": "annual",
          "day_count": "act/act",
          "yield_rate": 0.04})
bond = r.json()
print(bond["clean_price"], bond["dirty_price"], bond["bpv"])
# 102.4892 103.9413 -0.0257

03What "pricing a bond" actually computes

Pricing a fixed-rate bond means discounting its remaining cash flows — here the 5.00 coupons of July 2027, 2028 and 2029 plus the 100 principal at maturity — at the yield you supply, compounded at the coupon frequency. The API generates the coupon schedule backwards from maturity, applies the day count you name, and splits the present value into the three numbers markets actually use: the clean price (what is quoted), the accrued interest (coupon earned since the last payment), and the dirty price (what the buyer pays: clean + accrued).

The schedule and day-count bookkeeping is where spreadsheet bond math quietly goes wrong — which is why the request states every convention explicitly and the response repeats them back.

04Every field in the response

Sanity check the trio: modified duration × dirty price / 10,000 = 2.4748 × 103.9413 / 10,000 ≈ 0.0257 — the BPV, with the sign telling you the direction.

05Conventions

Two of these bite in practice. Settlement is taken literally — no implicit T+2, you pass the date the trade actually settles, which makes results reproducible instead of "correct on the day you ran it". And the YTM convention follows your request: it is compounded at the frequency you set and measured with your day_count, so an annual 4% and a semiannual 4% are different yields.

06Flat yield or your own curve

Replace yield_rate with a zero_curve (pillars in years + continuously-compounded zero rates) and the bond is discounted off the curve instead; you provide exactly one of the two. Need the curve itself from market quotes? That is POST /v1/curves/bootstrap. For the risk side of this same response — duration, convexity, BPV in depth — see bond duration, convexity and BPV over HTTP.