The United States needs to spend more on defense. Not a lot more, but more. And it needs to spend more in a predictable way. Today, as in the past, we’re swinging up and down. That’s wasteful, and it’s dangerous.

You may think that we spend enough, too much, or too little on defense already. But to make any of these arguments, you have to provide a yardstick or two. Here are mine.

Today, we spend just over $600 billion annually on defense. In 2015, that was 15 percent of all federal spending. In 1962, defense’s share was 50 percent. So the federal government is devoting a much lower share of its effort to defense than it used to.

Or you can look at defense as a share of our national output. Over the last 40 years, we’ve spent an average of 4.3 percent of our output on defense. If we spent that now, our defense budget would be roughly $775 billion, or $175 billion more than it actually is.

Here’s a useful way to think about it. Military spending is like insurance: If you don’t have it, things hurt a lot more when life goes bad. Would you spend $2,000 a year for full liability and replacement coverage on a $50,000 car? Well, that’s 4 percent of its value — about what we should spend on defense.

Too many people are still living in the post-9/11 world, where military spending did indeed ramp up. But since then, it’s ramped down, to the tune of a 25 percent real cut since 2011. And you can see this if you look at our forces, and our readiness.

The examples are legion. The Army in 2012 — just four years ago — had 45 brigade combat teams. Today it has 31. The average Air Force tactical aircraft is about 27 years old.

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Even worse — because it’s more subtle — is the readiness problem. The Marines have 276 F-18 Hornets: only 87 are currently able to fly. In theory, the Navy has 10 aircraft carriers: in practice, we can deploy only two. Across the board, training is down, while make-do and mend is up.

The answer is not to go hog wild on military spending. We don’t need to spend vastly more than we do now. Even returning to that average 4.3 percent share might be a bit much.

In our 2016 Index of U.S. Military Strength, my colleagues at the Heritage Foundation assessed our force posture and our needs. On the basis of our assessment, the U.S. should be spending around $700 billion annually. And, importantly, we should spend that much predictably.

When military spending surges and collapses — which is what we’ve done repeatedly since 1945 — bad things happen. No government agency, including the Pentagon, can spend piles of new money efficiently.

On the other end, when spending falls rapidly, training is cut and spare parts disappear. The force becomes less proficient, less ready — and the risks of big casualties during the next emergency rise.

So where does the money come from? It comes from cutting spending — including in the Pentagon, but not just there. By 2020, we could save $70 million merely by ending federal inspection of catfish. We can find that $100 billion if we try.

It’s possible for serious people to disagree about how much we should spend on defense. But the answer is not, as Sen. Dick Durbin (D-Ill.) put it recently, that new funding should be “fifty-fifty, defense and non-defense.” A dollar in this pile, a dollar in that pile is not a serious approach.

And that is what the United States needs: not binges, purges, or a dollar here and a dollar there. It needs a coherent assessment of our interests, what it takes to defend them, and how much those forces cost.

We should take defense seriously. And if we don’t notice that we’re not, the bad guys certainly will.

Ted R. Bromund is a senior research fellow in The Heritage Foundation’s Thatcher Center for Freedom.