# Fiber Optic Latency and Propagation Delay Calculator > Source: https://netray.co/tools/fiber-optic-latency-propagation-delay-calculator > Publisher: Netray (netray.co), AI and integration engineering for ERP and manufacturing systems. > Section: Free Tools ## Short answer Calculate one-way and round-trip delay over a fiber route, and find the longest route that meets a round-trip latency budget. ## Category industry-templates ## Industry Fiber Optics ## Audience - Network engineer - Fiber optic engineer - Solutions architect ## Tool **Kind:** calculator **Inputs:** #### Route length **Key:** length **Type:** slider **Default:** 80 **Min:** 1 **Max:** 400 **Step:** 1 **Unit:** km **Help:** Use the cable route length from the as-built records, not the straight-line distance. #### Group index of fiber **Key:** gi **Type:** number **Default:** 1.4682 **Min:** 1 **Step:** 0.0001 **Help:** About 1.467 to 1.469 for G.652 at 1550 nm. Use the datasheet value. #### Equipment latency per end **Key:** equip **Type:** number **Default:** 20 **Min:** 0 **Step:** 1 **Unit:** us **Help:** Transceiver, switch or muxponder latency at each end, per direction. #### Round-trip latency budget **Key:** budget **Type:** number **Default:** 1 **Min:** 0 **Step:** 0.1 **Unit:** ms **Outputs:** #### Delay per km **Key:** perkm **Expr:** gi / 0.299792458 **Format:** decimal **Description:** Microseconds per kilometre = group index / c, with c in km per ms. #### One-way fiber delay **Key:** oneway **Expr:** length * perkm **Format:** decimal **Description:** Microseconds. #### Round-trip time including equipment **Key:** rtt_us **Expr:** 2 * oneway + 4 * equip **Format:** decimal **Description:** Microseconds; equipment counted at both ends in both directions. #### Round-trip time **Key:** rtt_ms **Expr:** rtt_us / 1000 **Format:** decimal **Primary:** true **Description:** Milliseconds. #### Longest route within budget **Key:** maxkm **Expr:** max(0, (budget * 1000 - 4 * equip) / (2 * perkm)) **Format:** decimal **Primary:** true **Description:** Kilometres of fiber that fit inside the round-trip budget. **Chart:** #### Round-trip time vs route length (ms) **Type:** line **X:** length **From:** 1 **To:** 400 **Steps:** 40 **Series:** - rtt_ms **Disclaimer:** Excludes dispersion compensation modules, amplifiers with long coils and protocol or queuing delay. Measure real latency with a test set for SLA work. ## Content **Intro:** Light in single-mode fiber travels at about two thirds of its vacuum speed, so every kilometre adds close to 4.9 microseconds one way. This calculator converts route length and group index into delay and tells you how far you can go inside a round-trip budget. **Sections:** #### How to use the calculator **Body:** Enter the cable route length, the fiber group index and the latency of the equipment at each end. The result shows delay per kilometre, one-way fiber delay, round-trip time and the longest route that fits your round-trip budget. Use real route length from your as-built records because cables are longer than the straight line between sites by 20 to 50 percent in many metro and long-haul routes. **Bullets:** - Group index differs from the refractive index slightly; use the group figure - Equipment latency of coherent optics can be tens of microseconds - Dispersion compensating fiber adds its own length of delay #### The formula **Body:** Delay per kilometre is the group index divided by the speed of light in vacuum, which is 299,792.458 km/s. For a group index of 1.4682 this is 4.897 microseconds per km, often rounded to 5 microseconds per km. Round-trip time is twice the one-way delay plus equipment latency in each direction. The maximum route is the budget minus equipment latency, divided by twice the delay per kilometre. #### Worked example **Body:** For 80 km of fiber at a group index of 1.4682, one-way delay is 391.8 microseconds. With 20 microseconds of equipment latency at each end the round trip is 2 x 391.8 + 4 x 20 = 863.6 microseconds, or 0.864 ms. A 1 ms budget therefore supports a route of about 94 km before the budget is used, which is the kind of figure used when checking synchronous replication or high-frequency trading link limits. #### Common mistakes **Body:** Teams often quote 5 microseconds per km for the whole path while overlooking inline amplifiers with spooled gain fiber, dispersion compensating modules and regeneration sites. Others compare against an ICMP ping that includes switching and OS delay. For service-level work, measure with an RFC 2544 or Y.1564 test and use this tool only to plan and sanity-check results. **Cta:** Netray can bring route and circuit data from your ERP or inventory system into a single view when you need to document latency per circuit. **Faq:** #### How much latency does fiber add per kilometre? **Answer:** About 4.9 microseconds one way for standard single-mode fiber, so roughly 10 microseconds per km round trip. Hollow-core fiber is much faster, close to 3.4 microseconds per km. #### What is the group index of G.652 fiber? **Answer:** Typically about 1.467 to 1.469 at 1550 nm, slightly higher at 1310 nm. Use the value on the manufacturer datasheet when you have it. #### Why is my measured latency higher? **Answer:** Measured values include equipment, amplifiers, dispersion compensation and sometimes extra slack or loops in the cable that make the route longer than the records show. #### Does multimode fiber differ? **Answer:** Its group index is similar, about 1.48 for OM3 and OM4, so delay is around 4.9 to 5 microseconds per km. Multimode links are short, so the effect is usually negligible. ## Search terms fiber latency calculator, propagation delay fiber, latency per km fiber, fiber round trip time calculator, 5 microseconds per km, group index single mode fiber, rtt distance calculator, data center interconnect latency --- Talk to an engineer: https://netray.co/contact?intent=erp_ai | Machine catalog: https://netray.co/agent/catalog.json