Turning the iPhone 6s Into a Digital Scale

Ryan McLeod
The Startup
Published in
7 min readOct 27, 2015


September 9th, 2015

The iPhone 6s is announced and its brand new pressure sensitive hardware is unveiled. A simple but slightly vague API doc is published soon after. The question of calling the feature Force Touch or 3D Touch remains unresolved.

September 12th, 2015

My friend Chase McBride calls with a few tech questions (rough translation: startup idea). Chase and his friend Brice Tuttle have been talking over this idea of a scale app using 3D Touch. It’s the first I’ve heard of the idea. A quick peek through the APIs showed that we wouldn’t be able to simply place anything on the screen and get a force value. The only App Store friendly way to get a force value would be by touching the screen with a finger and even then the force returned would be the force directly under the finger.

We’re on the phone talking through this initial hurdle, when my excitement turns to concern. I start recalling other times Apple hasn’t exactly been stoked on novel uses of their devices’ sensors, but then I remember that despite this the App Store is speckled with creative workarounds including a panorama app that uses vibration to rotate the phone, magnetometer-based stud finders, camera/flash-based heart-rate monitors, and even Square’s ubiquitous headphone-jack card reader. A creative solution is waiting to be found. Creating a scale turns from impossible to a challenge.

September 21–24th, 2015

At this point we know that the force value returned is a float in the range of 0.00 to maximumPossibleForce (with 1.00 being an average touch). We still don’t know how to trigger a touch without a finger, but we have a few ideas.

I have a 5s I love, but decide if this is going to happen I need to preorder a 6s. We have roughly five days until the phone is in my hands, so what can we do in the meantime? We need to build a barebones testing app to hit the ground running and we need to come up with a way to weigh objects without touching the screen. Without that, we’ll be dead in the water.

With Chase and Brice in San Francisco, and me in San Luis Obispo we hop on FaceTime to channel our inner MacGyver and brainstorm a solution. We tried foil, watch batteries, apples, carrots, coins, even salami— at one point we had built a meat basket out of a piece of salami stuck to a jar lid with a needle as a conductive handle (pictured below). None of these solutions fit the trifecta of being convenient, common, and non-perishable.

Meat basket

September 24th, 2015

iPhone due to arrive tomorrow

As funny as the salami basket was, blindly coding something that seemed doomed to a greasy click-bait headline was burning up my morale.

We thought through the problem again: we needed an object that was conductive, had finger-like capacitance, formed a single finger-like touch point, was a household item, and could hold items to be weighed…

Conductive, capacitive, common, and curved to a single-point of contact.
A spoon was the perfect solution we had been looking for.

Gravity’s calibration screen

September 25th, 2015

Every car that drove by induced a mild panic attack. None of them were brown. When the UPS man finally arrived around dusk he barely made it across the street before I traded him my squiggle signature and ran back inside. 100g calibrated weight, jar of change, and digital scale all laid out in preparation, I downloaded the test app and begin recording the force values of different numbers of US nickels (~5g each) on a metal spoon.

0–10 nickels and then 10 nickels + 100g weight (on a metal spoon)

“Dude. It’s perfectly linear”

With the force values linearly correlated to weight, turning any force into a weight was going to be as simple as recording the force of known weights and creating a linear regression. It’d even be possible to use some statistics to predict how well the calibration went (there are many factors that can throw off a calibration). We opted to use coins for calibration, with a framework that made it easy to internationalize in the future.

September 29th, 2015

Waiting for Review

In less than four days we finished the app, tested our onboarding, put up a barebones website, got our marketing materials in line, planned a release strategy, and submitted to Apple.

October 7th, 2015


Gravity unfortunately got rejected for having a misleading description and we immediately knew why: There are a couple dozen “scale” apps on the app store. The thing is that 80% of them are joke apps, for entertainment purposes only and the other 20% try to weigh things using the tilt of your iPhone once it’s been balanced on top of an inflated bag and calibrated using a single coin. Gravity was most likely confused with the prank apps and rejected for claiming it was a real working scale.

Screenshot from an App Store search for “digital scale”

The Appeal

The very same day I rigged up a kitchen rack on some shoe boxes, made a little demo video, and filed an appeal to demonstrate that Gravity was a real working scale and may have been confused with prank apps.

The Demo Video

October 15th, 2015

No Scale Apps

To make a long story short the final answer over the phone was that the concept of a scale app was not appropriate for the App Store.

We were—and still are—bummed to say the least, but we understand some of the reasons Apple might not be allowing scale apps at this time.

Damage to the phone?

Maybe it’s because the screen could get damaged if people tried dropping heavy weights on their phone—thing is that the sensor won’t weigh beyond a maximum weight of ~385g (0.85 lbs) and you’d be hard pressed (har) to damage the screen with that little weight (Gravity also flashes a bright red warning). In addition to that it’s hard to balance heavy objects on a spoon, but then again people will be people and we completely understand why Apple didn’t advertise the 6s’s new water-resistant properties.

Misuse of the API?

The hardware behind 3D Touch is truly something to be excited about, and we have yet to see all of it’s creative applications explored. We understand that Gravity makes odd use of the API and 3D Touch sensor but know there are many published apps that use the iPhone’s hardware in novel ways; we also appreciate that these apps don’t always immediately make it onto the store.

Drug use?

Apple has a moral and ethical onus to make the right choices be it related to the confederate flag, changing drug laws, or using emoji to fight bullying, and we respect that. Some people immediately see a drug connotation with Gravity (the spoon definitely doesn’t help), but with ~1–3 grams of accuracy, no one is going to be weighing drugs with Gravity, but it still works well enough for my mom to determine how much postage goes on her letters to me.

So now what?

“An app store with over one-and-a-half million of the best apps available. That’s over one-and-a-half million hand-picked, awe-inspiring, just-plain-surprising, who-knew-a-phone-could-do-that apps.” —Apple’s Amazing Apps Ad

We have a strong respect for the subjective process Apple uses to maintain a selection of high quality apps and look forward to seeing other creative uses of 3D Touch, but do hope for a day when Gravity can be one of the hand-picked, who-knew-a-phone-could-do-that-apps anyone can download on the App Store and have in their pocket.

Want to know more?
Sign up for Gravity updates at gravity-scale.com.

Like this? You’ll probably love my other app: Blackbox

Published in #SWLH (Startups, Wanderlust, and Life Hacking)



Ryan McLeod
The Startup

Making @blackboxpuzzles – Apple Design Award cube carrier – was expelled from preschool for “defiance”.