What is the Acceleration Factor (AF) in Parabolic SAR?

The Parabolic SAR's Acceleration Factor (AF) starts at 0.02, increases with new price extremes, and accelerates the indicator’s sensitivity to trend changes, capping at 0.20.

Aug 05, 2025 at 10:00 pm

Understanding the Parabolic SAR Indicator

The Parabolic SAR (Stop and Reverse) is a technical analysis tool used primarily to determine the direction of an asset’s price movement and potential reversal points. Developed by J. Welles Wilder Jr., it is especially popular among traders engaged in trend-following strategies. The indicator appears as a series of dots placed either above or below the price candles on a chart. When the dots are below the price, it signals an uptrend; when above, a downtrend. The key mathematical component that governs the spacing and acceleration of these dots is the Acceleration Factor (AF).

Role of the Acceleration Factor in Parabolic SAR

The Acceleration Factor (AF) is a dynamic multiplier that increases as the trend extends over time, causing the SAR dots to converge more rapidly toward the price. This mechanism allows the indicator to "accelerate" in response to sustained price movement, hence the name. The AF starts at a base value—typically 0.02—and increases by a fixed increment, usually 0.02, each time a new extreme point (EP) is recorded. The EP is the highest high in an uptrend or the lowest low in a downtrend.

The AF directly influences how sensitive the Parabolic SAR is to price changes. A low AF results in wider spacing between SAR dots, making the indicator less likely to generate reversal signals. Conversely, a higher AF brings the dots closer to the price, increasing the chance of a trend reversal signal. This dynamic adjustment ensures that the SAR adapts to the momentum of the market.

Mathematical Formula and Calculation Steps

To compute the Parabolic SAR, the following formula is applied during an uptrend:

• SARₙₑw = SARₚᵣₑᵥ + AF × (EP - SARₚᵣₑᵥ)

And during a downtrend:

• SARₙₑw = SARₚᵣₑᵥ - AF × (SARₚᵣₑᵥ - EP)

The calculation process involves several precise steps:

• Initialize the AF at 0.02.
• Set the initial EP as the highest high (in an uptrend) or lowest low (in a downtrend) of the starting period.
• Use the starting SAR value, which is typically the prior extreme point.
• For each new period, update the SAR using the formula above.
• If a new high (in uptrend) or new low (in downtrend) is observed, increment the AF by 0.02, but only up to a maximum of 0.20.
• When the price crosses the SAR level, a trend reversal is signaled. At this point, the SAR, EP, and AF are reset for the new trend.

This iterative process ensures that the SAR dot trails the price with increasing speed as the trend persists, thanks to the escalating Acceleration Factor.

Impact of AF on Trading Signals

The value of the Acceleration Factor significantly affects the timing and frequency of trading signals. With a lower AF, such as 0.02, the SAR moves slowly and generates fewer reversal signals. This setting is suitable for strong, prolonged trends where whipsaws need to be minimized. However, in volatile or choppy markets, this may result in delayed exits.

When the AF is increased more rapidly or set with a higher step value, the SAR responds more aggressively. For example, if a trader modifies the increment to 0.03 instead of 0.02, the SAR will tighten around the price faster. This can lead to earlier exit signals and potentially lock in profits sooner, but it also increases the risk of being stopped out by short-term price fluctuations.

Traders often experiment with AF parameters to suit specific assets or timeframes. For instance, cryptocurrencies, known for their high volatility, might benefit from a capped AF of 0.20 to avoid premature reversals during sharp but temporary pullbacks.

Practical Example Using Bitcoin Price Data

Consider a scenario where Bitcoin enters an uptrend starting at $30,000. The initial SAR is set at $30,000, the AF at 0.02, and the EP at the first peak, say $31,000.

• On day two, if the price rises to $31,500 (a new high), the EP updates to $31,500, and the AF increases to 0.04.
• The new SAR is calculated as: SAR = previous SAR + 0.04 × ($31,500 - previous SAR).
• This process repeats each day the price makes a new high, with the AF rising by 0.02 until it hits 0.20.
• If Bitcoin then drops below the current SAR value, a reversal is triggered. The SAR flips above the price, the trend shifts to bearish, and the calculation restarts with a new EP (the recent high) and AF reset to 0.02.

This example illustrates how the Acceleration Factor enables the SAR to adapt to the momentum of Bitcoin’s price, providing dynamic support and resistance levels.

Customization and Parameter Adjustments

While the default AF settings are standardized, traders can adjust both the initial value and the increment to fine-tune the indicator. Some platforms allow users to input:

• A custom starting AF, such as 0.01 for slower acceleration.
• A variable increment, like 0.015, to moderate the rate of change.
• A different maximum AF, such as 0.15 or 0.25, depending on the asset’s volatility.

In crypto trading, where assets like Ethereum or Solana can exhibit rapid directional moves, a higher maximum AF may help the SAR keep pace. Conversely, for range-bound altcoins, a lower AF cap reduces false signals. These adjustments are typically made within the indicator settings of trading platforms like TradingView or MetaTrader.

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Frequently Asked Questions

What happens when the Acceleration Factor reaches its maximum value?

Once the Acceleration Factor hits its upper limit—commonly 0.20—it stops increasing, even if new extreme points are recorded. The SAR continues to follow the price at the maximum rate of acceleration, maintaining tighter tracking without further speeding up.

Can the Acceleration Factor decrease during a trend?

No, the Acceleration Factor only increases when new extreme points are reached. It does not decrease during an ongoing trend. It resets to the initial value only when a trend reversal is triggered and a new trend begins.

Is the Acceleration Factor the same across all assets?

While the default values are standardized, the effectiveness of the Acceleration Factor varies by asset. High-volatility cryptocurrencies may require different AF settings compared to stablecoins or traditional assets to optimize signal accuracy.

How does the AF differ from the Step and Maximum parameters in trading platforms?

The Step parameter refers to the initial and incremental value of the Acceleration Factor, while the Maximum sets the upper limit. Together, they define how the AF evolves. For example, Step = 0.02 and Maximum = 0.20 means the AF starts at 0.02, increases by 0.02 each time, and caps at 0.20.