How to read an exchange rate chart without over-reading it

Choose the window before you read the line

The same pair over seven days, three months and one year produces three different stories, and all three are true. A week shows noise. A quarter shows the current move. A year shows the trend and the range the pair has been living in. Deciding which question you are asking, before you look, is what stops a chart from confirming whatever you already believed.

A practical habit is to look at the longest window first. If a pair has spent a year between two levels, a week that runs up to one of them is a boundary, not a breakout. Reading the short window first makes every wiggle look like the beginning of something.

What percentage variation is actually measuring

The variation figure printed above most charts is the change between the first and last point of the window: last minus first, divided by first. It knows nothing about what happened in between. A pair that fell for eleven months and rebounded in the twelfth can show a variation close to zero over the year.

It is also entirely at the mercy of its start date. Shift the window by a few days across a sharp move and the same pair reads plus two percent or minus two percent. Whenever a variation figure is quoted at you, ask what window it covers before you give it any weight.

Read the vertical axis before you read the shape

Charting tools scale the vertical axis to fit the data rather than starting at zero. That is right for visibility and a trap for interpretation, because a pair that moved four tenths of a percent across the window will still fill the frame with peaks and valleys. The shape looks the same whether the total span is 0.4 percent or 40 percent.

Before drawing any conclusion, read the top and bottom values of the axis and work out the span between them. Then judge the shape against that number. Most of the crashes visible on a seven-day currency chart amount to a rounding difference on a restaurant bill.

Make sure you know which way is up

A rate is the price of one currency in another, so every chart has an implied direction. On a chart of euro to dollar, a rising line means the euro buys more dollars: the euro is stronger and the dollar weaker. Chart the same pair the other way round and the identical event produces a falling line.

This trips people up constantly, especially with currencies quoted in large numbers. A falling dollar-to-yen chart does not mean the yen is weak. It means a dollar buys fewer yen, so the yen has strengthened. When in doubt, read the line as how much of the second currency one unit of the first buys.

Why a one-day move is rarely a signal

Day-to-day movement in a major pair is mostly noise produced by liquidity, time zones and position adjustments. It reverses about as often as it continues, so treating it as information means acting on something with no predictive content.

Two mechanical details make single days even less trustworthy. Providers take their snapshots at different moments, so two sources can legitimately disagree about the same day. And currency trading effectively pauses over the weekend, so a gap on a Monday chart is often a calendar artefact rather than a move.

• A move worth noticing is large relative to the pair's own recent range, not relative to yesterday.
• Scheduled events, such as central bank meetings and inflation releases, explain a large share of the real moves. Check the calendar before inventing a story.
• A small disagreement between two sources is normal. A disagreement of whole percent usually means one of them is stale.

What a chart is genuinely good for

Three things. Knowing whether today's rate is high, low or unremarkable against the last few months. Seeing the range to expect if you convert the same pair regularly. And noticing when a pair has left the range it had been holding, which is the one single-chart observation that usually deserves attention.

If you are watching a pair because you intend to convert at some point, staring at a chart is a poor use of attention. All Currency Converter covers 7 days to 1 year with the minimum, maximum, average and percentage variation for a pair, and a rate alert sends a push notification when the pair crosses a level you set or moves by a percentage you choose.

None of this forecasts anything. Exchange rates over short horizons are close to unforecastable, and a chart is a record rather than a prediction. Use it to time a decision you have already made, not to make one.