love this calculator. hopeing that you will make an android ver. been having trouble figureing out my ridgelne length but this gave me a good place to start tyvm
Love it! Even bought your book! Would love an Android app.
Great news! The Android version is now available for both Google Play store and the Amazon app store.
Hej, there is an error in the JS code of this calculator. When I’m using the metric mode, none of the forces are displayed/updated. In the file hammock-hang-calculator.html in line 170 and line 171 the forces for tension and shear are expressed as variables, but they are implemented as function. The expression for the weight force in line 172 seems to be correct but isn’t executed because of the prior error.
Otherwise neat and easy to understand calculator. My first hammock has been shipped yesterday and I’m excited to try it out.
Thanks! I’ve fixed the code and it looks like it is working correctly now.
I bought your app and I got a question on a hennessey asym hammock isn’t the ridge line and hammock the same size? When I put in 15ft distance between anchors, Ridgeline and hammock length 100, preferred sit height 18, weight in hammock 170 lbs, thehang point says 41.1 inches. That is very low! If I put nothing for hammock length or ridgeline the hang point says NaN in?? Please advise
No, the ridgeline is not the same as the size of the hammock. The ridgeline is going to be shorter than your hammock. My Hennessy is only 9.5 ft long when laid down flat, but you’d have to detach the ridgeline in order to pull it out flat. The ridgeline is the cord that runs inside the bug net on the top where the pocket organizer lives. The ridgeline keeps the hammock in a curved shape.
Leave a number, even if zero (0) so the app doesn’t kick back a non-number error (NaN).
Love the calculator. Hope you make a android version. Android people hammock too.
I know, I know I’m looking for a developer. It’s not for lack of desire, but lack of funding.
Is the shear force total or per side? This is a very handy calculator. I’m trying to figure out how to effectively hang my hammock in my US style home, without risking a drop from any height.
I think what makes this confusing is the physics. There are a few forces at work here. First is the weight, or the vertical force. If you weigh 100 units, than each side would share 50 units of vertical force. If the ropes hung down at 90-degrees (straight down), the only force would be vertical. But there is also horizontal force, based on the angle of the hang. The calculator adds the horizontal force based on the angle, so each side will add up to more than half the weight that is in the hammock.
Half the occupant weight + horizontal force = total force on each side.
At 30-degrees, the horizontal force is half of the weight. Again, if the weight is 100:
50 (50% of occupant weight) + 50 (horz. force) = 100 units of force applied to _each_ side.
When you decrease the degree of the hang, you increase the horizontal force. Each side is still sharing an equal amount of vertical force, but they will _increase_ the horizontal force, meaning each side could bear _more_ force than what is being held in the hammock. This throws some folks off because it doesn’t seem to make sense that if I weigh 100 units, each side of the hammock suspension is also bearing 100 units. If you think in terms of “force” instead of “weight” it makes more sense (at least it does to me). It’s not that you magically star weighing _twice_ as much, but the forces are increasing.
In theory, a hammock strung perfectly horizontal (0-degree hang), the horizontal force would be infinite. This is why I encourage folks to hang their hammock at an angle.
Here’s where I’m confused… if the shear force is 300 units do I need two hooks that support 150 units each or two hooks that support 300 units each. I would hate for the hooks to fail and drop me 18 inches…plus I tend to wiggle/swing in my hammock. Falling is so not on my list of things to do. Especially unexpectedly while asleep. OW!
Was much easier in Mexico where the hooks are part of the cement wall and the hooks failing seems rather impossible!
I do appreciate your attempt to explain the physics to me and while I can grasp them theoretically, I’m just not wrapping my head around it completely. I would have probably just bought 2 hooks rated at 250 lbs and assumed I was okay, but then throwing the physics in… Argh! I haven’t had a reason to do physics for about 20 years… lol
Ha! I’m not good at physics either; I just spent a night in a Holiday Inn Express Seriously, I’ve had some help understanding this myself and I often have to refer to my notes to make sure I have it right.
What you are looking for in gear is “safe working load.” When hammocks are rated, for example, they take the breaking strength and then reduce that by a factor to get the safe working load. For example, a hammock with a weight limit of 400 lbs will likely have a breaking strength of 1,600 lbs. This is important because they consider the dynamic aspect of hammocks, just like climbing gear: as you move about in a hammock, you exert different amounts of force. This buffer gives a safe margin of error.
You typically want a safe working load of 4:1 or 5:1 or some go as far as 10:1. A 10:1 ratio is more common with more critical activities such as mountain climbing or hauling freight.
A hammock with a 4:1 safe working load factor that is rated at 250 lbs has a breaking strength of 1,000 lbs. This is sufficient for the type of activity in a hammock.
If the force is 300 lbs, you’ll want a minimum safe working load of 4:1, so you’ll want each component of your hammock system (hooks, ropes, hammock, etc.) to be safety rated above 300 lbs. It’s important to understand the difference between safe working load and breaking strength. Some suspension items, like webbing or rope, is often rated at breaking strength, so you’ll need to calculate and reduce them down to make sure they meet or exceed 300 lbs. If the hardware or soft good you purchase is “safety rated” at 300 lbs, than you should be fine. Manufacturers are required to list the ratio they used on the gear.
For inside my home, I purchased some 3/8 in steel eye bolts. They had a safety rating of 325 lbs and they’ve been working great!
Does this help?
That helps a lot. I’m dealing with an older Mayan string hammock, which I love and have no concerns over a sudden failure there. My only concern was with the bolts going into the stud. I could not imagine that I could unexpectedly break a steel hook (would expect signs of stress to show before failure) BUT when it comes to suspending myself from the wall, I’d prefer not to be stupid.
Sorry to be a pain, but your explanation of the division of the forces on the suspension lines I don’t quite get. If you have a total of 250 pounds (occupant plus the hammock, suspension and any additional gear), that is the total gravitational force acting on all points in the system. The only thing you can change is how this force is distributed (ie vertical or horizontal) . The only way you can add more overall force is if there is a ridgeline between the two hang points and this is pulled tight BEFORE adding the occupant weight. This additional force would then need to be added to the suspension line forces, but only if the ridgeline is attached on the lines below the hang points. If you are building in a safety factor by ensuring that each point will carry the total weight then I can fully appreciate that, but that should be stated in the graphic somewhere. Otherwise it is a great calculator and very helpful. Cheers.
Steve, you have a great question and it is one often brought up, so it isn’t a pain. I had the same questions before I understood how it works.
This is a question of physics related to practical trigonomic forces. The first thing to understand is that there is more than gravitational forces at play here. If it were just gravity, we wouldn’t need the calculator. The answer would be as simple as dividing the gravitational force in half (e.g., the weight in the hammock).
When you push or pull against a wall, you exert force against that wall. The wall also exerts force in the opposite direction in equal amounts. When your pull force exceeds the wall’s force (e.g., its rated strength), you can pull the wall down.
With trigonometry in play, you have to add up the total forces in play to get the final number. Gravity pulls, creating a downward force. The hammock also exerts a horizontal force/pull against the anchor point. Depending on the angle of the suspension, this horizontal force can be minimal (e.g., if the ropes are hanging straight downward), or the horizontal force could theoretically be infinite if the suspension was perfectly horizontal. Thankfully, it is impossible to achieve a perfectly horizontal force, for lots of reasons.
To find out the total force applied to the hammock we add up all the known forces.
Yes, you do divide the gravitational downward force, so a 100 lbs weight becomes 50 lbs of downward force on each side.
You also take the horizontal force and divide that in half. However, the horizontal force is variable, depending on the takeoff angle of the suspension. If it is too tight, the forces are stronger. This is why the “magic” 30-degree angle is recommended beyond making it easier to sleep on the diagonal. At 30-degrees, the horizontal force is roughly the same force as the downward force. This is why a 100 lbs occupant has 100 lbs of force on both sides of the hammock. We add the 50 lbs of downward force and the 50 lbs of horizontal force to each side. If the angle were different, the horizontal force would change, but the downward force would stay the same.
It seems almost counterintuitive that more force is being applied than there is weight in a hammock, as if you magically gain weight. You need to think in terms of forces not weight and it helps clarify the problem.
For example, a small pebble that weighs a few grams can exert more force than its weight if it is swung in a circle, say, in a sling. This centrifugal action applies more force than just the weight of the stone. If we could put a scale under the rock while it were spinning in a circle it would register as being “heavier” than if it were just hanging straight down.
The calculator is just for estimating, as I cannot predict all the forces at play, just those I can easily calculate for known values. For example, just sitting in a hammock adds some dynamic forces beyond a static weight. Speed and velocity of the sitting action can add more force than just the weight. Swinging in the hammock will add more force. When you consider all the forces that are or could be at play, you’ll see that this isn’t such a simple calculation at all, although the calculator itself is only doing the most rudimentary calculation.
Understanding these forces helps clarify to me why some hammocks rip apart when the occupant claims to weigh less than the hammock is rated for. This is also why folks recommend a safe working load limit by reducing the breaking strength by a factor of 5 or more to account for the unpredictable dynamic forces at play.
I hope this helps. Let me know if you have further questions.
I have a question on the suspension cord tension. If there are two hang points, how can each one be carrying the full weight? Or is that calculation the total between the two suspension lines? Same with the inward pull.
Hi – and wow……….all I want to do is sling a hammock in a shady spot ! Problem as I see it is that without an even length of rope on either end then everything changes and the calculations totally did my head in !! Does the calculator only work if the hammock is slung exactly in the middle of the supports ? Do I need to have the longer rope strung higher in the tree to still achieve a comfortable level ? I also plan this to be a comfortable 2 person hammock for lounging ( maybe the magic 30 degrees changes ?? ) Any suggestions as to a good quality, affordable and durable hammock that fits the bill here ? Thanks for doing my head in – as a previous post said, ” falling is not on my to do list !! “. Cheers, Todd.
Todd, not to worry! This calculator is more for estimating, as I mentioned in another reply, there are a lot of dynamic forces I cannot calculate for. This tool is best used as a way to figure out approximately how high to attach your hammock. For example, if you know how long your hammock is and how far apart the anchors are (e.g., inside a room) you can punch in the numbers and get the height where to set the hooks (or tree straps, etc).
The force calculation may change if the hammock is not level and even, but I suspect that it’s not far off. I think more depends on how the weight is distributed in the hammock.
As for a good two person hammock, I suspect you are looking for something to hang in the backyard? For casual lounging any hammock labeled “double” will do, but they won’t be cozy for sleeping long term unless you both like conjoined sleeping.
A great hammock for lounging is the queen or ‘matrimonial’ Mayan hammock. Bar none the most comfortable hammock. Authentic Mayan hammocks sold from Hammock Rada for example are amazing. Too heavy for backpacking, but backyard hangs or car camping are perfect.
For camping, I recommend getting separate hammocks.
Hi, Thanks for a fantastic resource. Could you clarify one thing for me please?
Is the “suspension length” number for each side, or is it a total length for both sides of the suspension?
Deon, the length is for each side.
It’s amazing what we can do with math. I wondered growing up how these things might be useful. Its a good tool thanks.
well derek, i guess my question is i’m not sure why i’m not getting the ultimate hang…but maybe it’s cause i’m being careless…
usually what i do is:
- 1st measure the distance between the trees and input that as the distance between anchors
- 2nd, according to the hang point the app gives me, hang hammock from that height
- 3rd, adjust suspension length
i may not use exact numbers but pretty close, and it usually ends up falling too low to the ground or too tight to be able to lay down flat….,
sometimes i feel like i may be choosing tree that are too close to each other, or there’s some other variable…i’m relatively new to this, but have hung maybe 8 times or so
Possibly a dumb question. Can I use this even if I don’t use a ridgeline? I am planning an inside hang and don’t want a ridgeline in the way.
Yes! The ridge line is not a necessity. Use as normal and remove the ridge line length.
Hi! What about a hammock with spreader bars ?
Very good question. If you’re looking for help with rope hammocks, in afraid I cannot help you. I’m not a fan of these tippy contraptions and would prefer if the world was rid of them. Seriously though, I’m not planning to support those hammocks but the general rules apply. I do need to update the calculator for bridge style hammocks that have spreader bars. It gets tricky, though, because if the three commercially available bridge hammocks, they have different requirements.
Since we like to hang the foot end a little higher than the head end….do you have a suggestion for the difference?
This is an interesting request that I’m considering when updating the app. It’s really a subjective measure based on your own preferences. From the baseline, you could drop the head end lower, or raise the foot end higher as both achieve the same result. Anything from six inches to a foot is about all that’s necessary.
Thanks for the reply!