Fibonacci Arc Calculator

A Fibonacci arc is a curved variant of a support or resistance level: instead of a flat line or a straight diagonal like the Fibonacci Fan Calculator, it draws a curve centered on the swing's end, with three radii set by the 38.2%, 50%, and 61.8% Fibonacci ratios of the swing's price range.

Because a circle lives in two equal dimensions but price and time (bars) aren't naturally the same unit, this calculator makes the price-per-bar scale an explicit, editable input rather than hiding that assumption the way most charting platforms do behind their own axis scaling.

How does this calculator work?

Enter the swing low and swing high, the trend direction, and how many bars the swing took from start to end.

The calculator suggests a price-per-bar scale automatically, swing price range ÷ swing duration in bars, treating the swing as a roughly 45-degree move. You can override this to match your own chart's scaling if you prefer.

Each arc's radius (in price units) is the ratio times the swing's price range: for example, the 61.8% arc has a radius of 0.618 × range. All three arcs are centered on the end of the swing, the high in an uptrend or the low in a downtrend.

To find where an arc currently sits, enter how many bars past the end of the swing you want to check. That distance is converted to price-equivalent units using the scale, then standard circle geometry (price = center ± √(radius² − distance²)) gives the upper and lower price the arc crosses at that point in time. If the queried point is farther out than an arc's radius, that arc has already collapsed to its center price and no longer applies.

A chart plots each arc's upper and lower band out to the smallest arc's own radius, the range where all three arcs still have a real width, so you can see them curving inward and the narrowest one closing to a point.

Worked example

The same $50-to-$100, 10-bar uptrend swing used in the other Fibonacci calculators, checking where the arcs sit 3 bars past the end of the swing.

38.2% arc

$88.18 to $111.82

50% arc

$80.00 to $120.00

61.8% arc

$72.99 to $127.01

How the numbers work

The suggested scale is $50 ÷ 10 bars = $5 per bar, so 3 bars forward is equivalent to $15 in the circle's geometry.

The 50% arc has a radius of 0.5 × $50 = $25. With the query point $15 away (in price-equivalent terms) from the $100 center, the arc's actual price offset at that point is √(25² − 15²) = $20, giving a band from $80 to $120.

Arcs curve inward as time passes, unlike the flat retracement levels or straight fan lines, they eventually collapse back to the center price entirely (once the query distance exceeds the radius). That curvature is the whole point: an arc gets tighter around the center price the farther out in time you look, reflecting the idea that a level's relevance fades with time.