Composite & famousAccelerator Oscillator · AC
Bill Williams' measure of whether momentum itself is speeding up or slowing down.
Works in most conditionsEngine-computed on a fixed sample series
What it is
Bill Williams' Accelerator Oscillator measures not momentum itself but the acceleration of momentum — whether the force behind price is speeding up or slowing down. It answers a subtle question: is the current move gaining energy or quietly running out of it, even before price or ordinary momentum shows the change? Williams' insight was that force (acceleration) changes direction before speed does, the way a car eases off the gas before it actually slows, so the tool is designed as an early-warning gauge. It is drawn as a histogram of colored bars oscillating above and below a zero line, and it is meant to be read one layer earlier than the Awesome Oscillator it is built from.
How it's calculated
The Accelerator starts from Bill Williams' Awesome Oscillator, which is the difference between a 5-period and a 34-period simple moving average of each bar's median price, the median being (High + Low) / 2. The Accelerator then subtracts a 5-period simple moving average of the Awesome Oscillator from the Awesome Oscillator itself: AC = AO − SMA(AO, 5). Conceptually, the Awesome Oscillator is a proxy for momentum and the 5-period average of it is a proxy for the recent baseline of that momentum, so their difference isolates the momentum that sits above or below its own recent norm — the acceleration. The histogram is then colored: a bar is green when it is higher than the previous bar and red when it is lower. Because it is a derivative of a derivative, the line is inherently jumpy and centers on zero.
Reading it, step by step
Two things matter on the Accelerator: which side of zero the bar sits on, and its color. A green bar means acceleration is increasing at that moment and a red bar that it is decreasing, regardless of whether the histogram is above or below zero. Because acceleration leads speed, a shift from red to green on the Accelerator frequently appears before the matching turn shows up in the Awesome Oscillator, giving an early heads-up. Bars above zero turning green signal building bullish force; bars below zero turning green hint that bearish force is fading and a bottom may be forming. The zero line is the boundary between net positive and net negative acceleration, but Williams stressed color over position for timing.
Best timeframes and settings
The Accelerator uses fixed Bill Williams parameters — 5 and 34 for the underlying Awesome Oscillator and a 5-period smoothing — and these are almost never changed, because the whole Williams system is calibrated around them. It can be applied on any timeframe from 1-minute scalping charts to weekly position charts, but its jumpiness makes it most useful as a confirmation layer on the swing and intraday timeframes where the underlying oscillator has room to develop clean swings. On very fast charts the color flips so often that the early-warning value drowns in noise. Because the parameters are locked, the responsiveness-versus-noise trade-off is managed by choosing the timeframe rather than the settings. Traders who want a smoother read simply move up a timeframe rather than lengthening the averages.
When and where to use it
The Accelerator is a context and timing tool inside the broader Bill Williams trading system, not a standalone signal generator. It is most valuable when a trend is already identified by the Alligator and you want the earliest possible read on whether the current push has force behind it. It works on any liquid instrument, but like all momentum derivatives it is happiest in markets that actually swing rather than grind sideways. In a flat, directionless market the histogram flickers red and green around zero endlessly, and acting on those flickers is a fast way to overtrade. Use it to confirm and time entries in an established trend, and at least discount it when no trend is present.
Strategies that use it
Williams' own rule set keys off color and zero. To buy, he required the histogram to be green; if the current bar is above zero he wanted at least two consecutive green bars, and if it is below zero he wanted three consecutive green bars before entering, the extra bar demanded because buying below zero fights the prevailing force. The short rules mirror this with red bars. A practical layered strategy stacks the Accelerator on the Awesome Oscillator and the Alligator: enter only when the Alligator is awake in the trade direction, the Awesome Oscillator agrees, and the Accelerator has just turned its color in that direction as the timing trigger. A simpler momentum-fade variant watches for the Accelerator to make a lower high while price makes a higher high, using that divergence as an early warning to tighten stops rather than to reverse outright.
Combining it with other indicators
The Accelerator was designed to be used with its siblings — the Awesome Oscillator, the Alligator and the Gator Oscillator — and it is most coherent inside that toolkit. The Alligator supplies the trend context that tells you whether an Accelerator signal is worth taking, and the Awesome Oscillator supplies the momentum layer the Accelerator is one step ahead of. Outside the Williams family, it complements a trend filter such as ADX, which keeps you from acting on color flips during a range. Because it is an early-warning gauge, it pairs well with a slower confirmation like a MACD signal-line cross: the Accelerator turns first, the MACD confirms, and the two together reduce false starts. Pairing it with price structure — a break of a swing level — turns its early signal into an actionable trigger.
Where it fails
As a derivative of a derivative, the Accelerator is the jumpiest tool in the Williams kit, and its worst habit is producing color changes that lead nowhere, especially near zero in a quiet market. Traders who treat every green-to-red flip as a signal will be whipsawed relentlessly, because the tool was explicitly designed as an early warning, not a trigger. Its early nature is double-edged: being ahead of momentum means it also gives more false alarms than the momentum itself. It carries no fixed overbought or oversold level, so extremes have to be judged against the instrument's own recent history rather than an absolute threshold. The way to avoid the traps is to demand trend context, require Williams' multi-bar confirmation, and never trade the Accelerator alone.
A worked example
Suppose a currency pair is in an uptrend, the Alligator's lines are fanned upward, and price pulls back for several sessions. The Awesome Oscillator is still positive but flattening, and the Accelerator histogram, sitting just above zero, prints two consecutive green bars after a run of red ones as the pullback ends. Under Williams' rule — green and above zero, two green bars — a trader takes the long on the second green bar at 1.1050, reading the color change as force returning to the up move ahead of any fresh price high. Price resumes its climb to 1.1150 over the next week, and only when the Accelerator rolls over to a string of red bars while price stalls does the trader tighten the stop. The example shows the tool's role: it timed the re-entry a bar or two before the Awesome Oscillator confirmed, but it worked only because the trend context was already bullish.