340 and 400 spine arrows are the two most common arrow spines used. Often for most bow hunters or archers, a 340 or 400 spine arrow tuns best out of their bow. Because these two spines are often used a lot it would only make sense to know the difference.  

Here is the short answer:

If you shoot a heavier draw weight, 60-70 pounds, with an arrow over between 27-30 inches, or if you want to build a heavier arrow, you will want a 340 spine arrow. If you shoot from 50 to 40 pounds or shoot an arrow under 26 inches, in most cases you will want to shoot a 400-spine arrow.

Now that we have looked at the short answer, we can look in depth at what factors will lead you to desire a stiffer or less stiff spine.

340 vs. 400 spine chart

The easiest way to determine what spine you need is to find your bow setup on a graph and then find a suitable spine stiffness. 

	Arrow length	25”	26”	27”	28”	29”	30”	31”	32”
Draw weight
40-45		500	500	500	500	500	400	400	400
45-50		500	500	500	400	400	400	400	400
50-55		500	400	400	400	400	400	340	340
55-60		400	400	400	400	340	340	340	300
60-65		400	400	340	340	340	340	300	300
65-70		340	340	340	300	300	300	250	250
70-75		340	300	300	300	250	250	250	250

Keep in mind that if you have 150 plus grain to total point weight (total point weight meaning inserts and field point/broadhead total weight), you will want to go one spine size lower. This means that if you have a 60-pound draw with a 28-inch arrow and you have 150 grains of point weight, instead of a 340-spine arrow, you will want a 300-spine arrow.

Which spine is better and when 

Which spine you will want is dependent on a few factors, so let’s see what those factors are.

The most significant variable on which spine size you will want is your draw weight. The heavier the draw weight you have, the stiffer spine you will need. So if you draw from 60-70 pounds, generally, you will want 340 spine. If you shoot 50-60 pounds, you will want a 400 spine.

If you shoot a longer arrow, you will want a stiffer spine. The more length of arrow you have, the more flex that can be put on the shaft in return, you will need a stiffer arrow. Even if you shoot a light poundage, like 50 pounds, but have a long arrow, 30+ inches, you will want a 340 spine. However, if you have a short arrow, under 25 inches, even if you shoot a 70 pounds bow, you will want a 340 spine.

A less accounted-for factor that affects arrow spine is front-end weight. The more weight you put at the end of your arrow, the stiffer of a spine you will want. This is because the extra weight on the end of your arrow adds extra bend to it. So if you need a 400 spine arrow normally but want to add a 50-grain insert instead of a standard 14-grain insert, you should get a 340 spine.

Conclusion 

340 spine and 400 spine have different uses that are very important. Before you choose one spine over the other, you should consider your draw weight, your arrow length, and how much total point weight you plan on having. If you account for those three variables, your bow will tune great.