Percentage Trailing Stop

A fixed-percentage stop that ratchets with the trend and flips to the opposite side on a close-through. The trail width is purely proportional to the latest price, so it scales naturally across instruments without re-tuning — a 5% trail behaves the same on a $10 stock and a $1000 stock.

Quick reference

Item	Value
Family	Trailing Stops
Input type	f64 (close)
Output type	f64 — the active stop level
Output range	unbounded (price-units)
Default parameters	percent = 5.0 (PercentageTrailingStop::classic())
Warmup period	1
Interpretation	Stop level; flips to opposite side on close-through

Formula

step = |close| · percent / 100

long:   stop_t = max(stop_{t−1}, close − step)   while close ≥ stop_{t−1}
short:  stop_t = min(stop_{t−1}, close + step)   while close ≤ stop_{t−1}

flip-to-long  on a close > prev short-stop  ->  stop = close − step
flip-to-short on a close < prev long-stop   ->  stop = close + step

The first input seeds a long stop percent below price. Long stops ratchet up monotonically; short stops ratchet down monotonically. See crates/wickra-core/src/indicators/percentage_trailing_stop.rs.

Parameters

Name	Type	Default	Constraint	Description
percent	f64	5.0	finite, > 0	Trail width as a percentage of the latest close.

PercentageTrailingStop::new returns Error::NonPositiveMultiplier for non-finite or non-positive percent. PercentageTrailingStop::classic() returns the 5.0% factory.

Inputs / Outputs

Indicator<Input = f64, Output = f64>. Python: PercentageTrailingStop(percent).batch(close) returns a 1-D np.ndarray — no warmup NaNs because the indicator emits on the first input. Node: same shape; update(close) returns number.

Warmup

warmup_period() == 1. The first input seeds a long stop at close · (1 − percent/100) and the indicator emits immediately.

Edge cases

• First bar. Always seeds long with stop percent below seed close.
• close == 0.0. step == 0, so the stop equals the close. The next non-zero close triggers a normal flip evaluation.
• Negative close. |close| is used to compute step, so the trail width stays positive for short-side equity-equivalents (futures with negative prices, spread products). The indicator treats the trail conceptually as one-sided per state, not on a signed axis.
• Flat tape. A constant close holds the stop fixed; no flip.
• Reset. reset() clears prev_stop to None and resets the side flag to long.

Examples

Rust

use wickra::{BatchExt, Indicator, PercentageTrailingStop};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let prices: Vec<f64> = (0..40).map(|i| 100.0 + f64::from(i)).collect();
    let mut p = PercentageTrailingStop::classic();
    let out = p.batch(&prices);
    println!("row 10 = {:?}", out[10]);
    Ok(())
}

Python

import numpy as np
import wickra as ta

prices = 100 + np.arange(40, dtype=float)
p = ta.PercentageTrailingStop(5.0)
out = p.batch(prices)
print('warmup:', p.warmup_period())  # 1
print('row 10:', out[10])

Node

const wickra = require('wickra');

const p = new wickra.PercentageTrailingStop(5.0);
const prices = Array.from({ length: 40 }, (_, i) => 100 + i);
const out = p.batch(prices);
console.log('row 10:', out[10]);

Streaming

use wickra::{Indicator, PercentageTrailingStop};

let mut p = PercentageTrailingStop::classic();
let close_stream: Vec<f64> = Vec::new(); // your live close-price feed
for close in close_stream {
    let stop = p.update(close).unwrap();
    // The indicator's internal `long` bit is private; infer from stop position.
    let side = if close > stop { "long" } else { "short" };
    println!("close={close:.4}  stop={stop:.4}  side={side}");
}

Interpretation

• Trail width is proportional to price. 5% means $5 on a $100 stock and $50 on a $1000 stock; the indicator self-scales.
• Symmetric flip behaviour. Unlike one-sided trailing exits (e.g. YoyoExit), this indicator flips both ways: a long close-through opens a short, and vice versa.
• Vs ATR trailing stops. Percentage trails are constant in relative terms; ATR trails are constant in volatility-scaled terms. Pick percentage when the instrument's volatility is roughly proportional to its price (most equities), ATR when volatility regime drifts independently (commodities, futures rolls).

Common pitfalls

• Choosing percent without backtest evidence. 5% is a default, not a recipe. Intraday systems may need 0.5% - 1.0%; weekly swing systems may need 8% - 15%.
• Forgetting the flip. Many traders think of "trailing stop" as one-sided; this indicator goes short on every long-side stopout. If you only want long exits and no short positions, use YoyoExit instead.
• Gap risk. A gap-down through the long stop fills you at the gap close — the percentage trail provides no buffer.

References

• Wilder's New Concepts in Technical Trading Systems (1978) discusses parameter-based stops; the explicit "percentage trail" pattern dates to broker-side software in the 1980s.

See also

• AtrTrailingStop — volatility-scaled cousin.
• StepTrailingStop — discrete-grid cousin.
• SuperTrend — (HL2 ± mult·ATR)-based flip stop.
• Indicators-Overview — full taxonomy.