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The Taylor Rule: A Rough Map for Where Interest Rates Should Be

The Taylor rule is a simple formula that suggests what a central bank's policy rate should be, given inflation and economic slack. It does not predict decisions, but it offers a benchmark.

In its basic form, the rule says the policy rate should equal the neutral real rate plus current inflation, plus half the gap between inflation and its target, plus half the output gap, which measures how far the economy is from its potential. If inflation is above target and the economy is running hot, the rule calls for a higher rate.

For example, with a neutral real rate of 0.5 percent, inflation at 4 percent against a 2 percent target, and no output gap, the rule gives 0.5 plus 4 plus half of 2, which is 5.5 percent. If the actual policy rate is 4 percent, the central bank appears behind the curve, and markets may expect more hikes.

The rule has limits. Estimates of the neutral rate and output gap are uncertain, and different versions give different answers. Central banks do not follow it mechanically.

Still, comparing actual rates with rule-based estimates provides context. A large gap between the two suggests a likely direction for policy.

Many research departments publish updated Taylor rule estimates, and they are discussed in commentary.

Practical step: compute the simple Taylor rule rate for one economy with current inflation, and compare it with the actual policy rate.

The neutral rate guess is doing most of the work in the formula. Change it by one percent and the answer changes by one percent, so treat the output as a rough zone, not a number. A good habit is to calculate it with two or three neutral rate assumptions, say 0.5, 1 and 1.5 percent, and see whether the actual policy rate sits inside or outside that whole range. If the policy rate is below all of them while inflation is above target, the case for tighter policy is strong. If it sits somewhere in the middle, the formula doesn't tell you much, and the data and central bank language matter more.

Nice way to frame behind the curve talk in the news. When commentators say that, they usually mean the policy rate is well below some version of this rule. The reverse matters too. When the policy rate is clearly above what the rule suggests, people start talking about policy being too tight, and markets begin pricing cuts. For currency traders, the gap between the rule and the actual rate across two countries can help explain longer swings in a pair. If one central bank looks well behind the curve and the other looks ahead of it, the currency of the one that has to catch up often strengthens over the following months.

Fairman wrote: The rule has limits. Estimates of the neutral rate and output gap are uncertain, and different versions give different answers.

The inflation input matters almost as much as the neutral rate. For the Fed, using core PCE instead of headline CPI can shift the answer by half a point or more, since the two measures often differ by that much in either direction. I'd run the calculation with the measure the central bank itself targets, which for the Fed is PCE and for the ECB and the Bank of England is the headline consumer price index, and treat the result as a range across those choices.