diff --git a/Cargo.lock b/Cargo.lock
index fce523b..913ef75 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -778,6 +778,12 @@ dependencies = [
  "digest 0.11.3",
 ]
 
+[[package]]
+name = "humantime"
+version = "2.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "15cdd26707701c53297e2fa6afb323d55fbc1d0810c3aec078ae3ef0424c3c15"
+
 [[package]]
 name = "hybrid-array"
 version = "0.4.15"
@@ -893,6 +899,7 @@ dependencies = [
  "ctrlc",
  "futures-core",
  "hex",
+ "humantime",
  "irtt-client",
  "irtt-proto",
  "irtt-server",
diff --git a/crates/irtt-app/Cargo.toml b/crates/irtt-app/Cargo.toml
index 4328ac0..2ffafa7 100644
--- a/crates/irtt-app/Cargo.toml
+++ b/crates/irtt-app/Cargo.toml
@@ -42,9 +42,9 @@ doc = false
 
 [features]
 default = ["client", "server", "tui"]
-client = ["dep:tokio", "dep:irtt-client", "irtt-client/ancillary", "irtt-client/tokio", "dep:irtt-proto", "dep:irtt-stats"]
+client = ["dep:humantime", "dep:tokio", "dep:irtt-client", "irtt-client/ancillary", "irtt-client/tokio", "dep:irtt-proto", "dep:irtt-stats"]
 server = ["dep:irtt-server", "dep:tokio"]
-tui =["dep:tokio", "dep:futures-core", "dep:irtt-client", "irtt-client/ancillary", "irtt-client/tokio", "dep:irtt-proto", "dep:irtt-stats", "dep:ratatui", "dep:crossterm", "dep:chrono"]
+tui =["dep:humantime", "dep:tokio", "dep:futures-core", "dep:irtt-client", "irtt-client/ancillary", "irtt-client/tokio", "dep:irtt-proto", "dep:irtt-stats", "dep:ratatui", "dep:crossterm", "dep:chrono"]
 
 [dependencies]
 chrono = { version = "0.4.45", default-features = false, features = ["clock"], optional = true }
@@ -53,6 +53,7 @@ clap-num = "=1.2.0"
 crossterm = { version = "0.29.0", features = ["event-stream"], optional = true }
 futures-core = { version = "0.3.34", default-features = false, optional = true }
 hex = "0.4.3"
+humantime = { version = "2.4.0", optional = true }
 ctrlc = { version = "3.5.2", features = ["termination"] }
 irtt-client = { workspace = true, optional = true }
 irtt-proto = { workspace = true, optional = true }
diff --git a/crates/irtt-app/src/cmd/client/args.rs b/crates/irtt-app/src/cmd/client/args.rs
index 71e121b..f936f0a 100644
--- a/crates/irtt-app/src/cmd/client/args.rs
+++ b/crates/irtt-app/src/cmd/client/args.rs
@@ -40,8 +40,8 @@ pub struct ClientArgs {
         long,
         default_value = "10s",
         value_parser = parse_test_duration,
-        help = "Test duration; use 0 for continuous mode",
-        long_help = "Test duration; use 0 for continuous mode.\n\nFinite runs retain exact statistics for final summaries. Continuous mode uses bounded-memory running statistics and prints a final summary when interrupted; --targets-stdin also prints the final retained summary on EOF."
+        help = "Test duration (e.g. 2.5s, 1m30s, 1h); use 0 for continuous mode",
+        long_help = "Test duration (e.g. 2.5s, 1m30s, 500us, 100ns, 1h); use 0 for continuous mode.\n\nFinite runs retain exact statistics for final summaries. Continuous mode uses bounded-memory running statistics and prints a final summary when interrupted; --targets-stdin also prints the final retained summary on EOF."
     )]
     pub duration: Duration,
 
diff --git a/crates/irtt-app/src/cmd/tui/args.rs b/crates/irtt-app/src/cmd/tui/args.rs
index a99675f..8a7560d 100644
--- a/crates/irtt-app/src/cmd/tui/args.rs
+++ b/crates/irtt-app/src/cmd/tui/args.rs
@@ -35,8 +35,8 @@ pub struct TuiArgs {
         long,
         default_value = "0",
         value_parser = parse_test_duration,
-        help = "Test duration; use 0 for continuous mode",
-        long_help = "Test duration; use 0 for continuous mode. The TUI defaults to continuous mode."
+        help = "Test duration (e.g. 2.5s, 1m30s, 1h); use 0 for continuous mode",
+        long_help = "Test duration (e.g. 2.5s, 1m30s, 500us, 100ns, 1h); use 0 for continuous mode. The TUI defaults to continuous mode."
     )]
     pub duration: Duration,
 
diff --git a/crates/irtt-app/src/shared/client/args.rs b/crates/irtt-app/src/shared/client/args.rs
index cbdc8fe..1bcbf05 100644
--- a/crates/irtt-app/src/shared/client/args.rs
+++ b/crates/irtt-app/src/shared/client/args.rs
@@ -18,7 +18,7 @@ pub struct CommonClientArgs {
     #[arg(long, conflicts_with_all = ["ipv4", "ipv6"], long_help = "Probe available IPv4 and IPv6 paths separately for each hostname. Labels gain /v4 or /v6; IP literals remain single targets. Families are discovered when a target is added; reconnects resolve the original hostname again within the chosen family.")]
     pub dual_stack: bool,
 
-    /// Probe interval.
+    /// Positive probe interval (e.g. 2.5s, 1m30s, 500us, 100ns, 1h).
     #[arg(long, default_value = "1s", value_parser = parse_duration)]
     pub interval: Duration,
 
@@ -200,62 +200,18 @@ fn parse_bind_addr(input: &str) -> Result<SocketAddr, String> {
 }
 
 pub fn parse_duration(input: &str) -> Result<Duration, String> {
-    let (number, unit) = split_duration(input)?;
-    let value: u64 = number
-        .parse()
-        .map_err(|_| format!("invalid duration value {input:?}"))?;
-    if value == 0 {
+    let duration = parse_test_duration(input)?;
+    if duration.is_zero() {
         return Err("duration must be greater than zero".to_owned());
     }
-    match unit {
-        "ms" => Ok(Duration::from_millis(value)),
-        "s" => Ok(Duration::from_secs(value)),
-        "m" => value
-            .checked_mul(60)
-            .map(Duration::from_secs)
-            .ok_or_else(|| "duration is too large".to_owned()),
-        _ => Err(format!(
-            "unsupported duration unit {unit:?}; use ms, s, or m"
-        )),
-    }
+    Ok(duration)
 }
 
 pub fn parse_test_duration(input: &str) -> Result<Duration, String> {
     if input == "0" {
         return Ok(Duration::ZERO);
     }
-    let (number, unit) = split_duration(input)?;
-    let value: u64 = number
-        .parse()
-        .map_err(|_| format!("invalid duration value {input:?}"))?;
-    if value == 0 {
-        return Ok(Duration::ZERO);
-    }
-    match unit {
-        "ms" => Ok(Duration::from_millis(value)),
-        "s" => Ok(Duration::from_secs(value)),
-        "m" => value
-            .checked_mul(60)
-            .map(Duration::from_secs)
-            .ok_or_else(|| "duration is too large".to_owned()),
-        _ => Err(format!(
-            "unsupported duration unit {unit:?}; use ms, s, or m"
-        )),
-    }
-}
-
-fn split_duration(input: &str) -> Result<(&str, &str), String> {
-    let split = input
-        .find(|ch: char| !ch.is_ascii_digit())
-        .ok_or_else(|| "duration must include a unit: ms, s, or m".to_owned())?;
-    let (number, unit) = input.split_at(split);
-    if number.is_empty() || unit.is_empty() {
-        return Err("duration must include a positive value and unit".to_owned());
-    }
-    if number.starts_with('-') {
-        return Err("duration must be greater than zero".to_owned());
-    }
-    Ok((number, unit))
+    humantime::parse_duration(input).map_err(|error| error.to_string())
 }
 
 pub fn parse_length(input: &str) -> Result<u32, String> {
diff --git a/docs/irtt-client.md b/docs/irtt-client.md
index 917f68d..1b6b647 100644
--- a/docs/irtt-client.md
+++ b/docs/irtt-client.md
@@ -174,6 +174,12 @@ expansion: a hostname producing both families consumes two target slots.
 
 ## FINITE VERSUS CONTINUOUS OPERATION
 
+`--duration` and `--interval` accept fractional and composite durations, such as
+`2.5s`, `1m30s`, and `1h`, as well as `ms`, `us` (or `µs`), and `ns` units
+(e.g. `500us` or `100ns`). A nonzero value requires a unit. The interval must
+be greater than zero; a zero test duration, including `0s`, selects continuous
+mode.
+
 `--duration` controls the run length:
 
 - Any nonzero duration (default `10s`) is a **finite** run: probing stops
diff --git a/docs/irtt-tui.md b/docs/irtt-tui.md
index 98cfe5b..4c6bc2d 100644
--- a/docs/irtt-tui.md
+++ b/docs/irtt-tui.md
@@ -40,7 +40,10 @@ See the client manual's address-family section for discovery errors and labels.
 
 Unlike `irtt-client`, **the TUI defaults to continuous mode**
 (`--duration 0`): it runs until you quit. Pass an explicit `--duration` for
-a finite run:
+a finite run. Both `--duration` and `--interval` use the
+[client duration syntax](irtt-client.md#finite-versus-continuous-operation),
+including fractional values (`2.5s`), composite values (`1m30s`), and units
+from nanoseconds (`ns`) through hours (`h`):
 
 ```sh
 irtt-tui <server> --duration 30s