What the calculator is actually solving for
Plantar fasciitis relief from an insole comes from two things: arch support that reduces strain on the plantar fascia, and a deep heel cup that stabilises the heel fat pad. Cushioning feels nice in the shop but does little to offload the fascia, and a soft insole that collapses under your weight loses whatever support it started with.
So the useful question is not "hard or soft" but "how much arch height, and how firm a shell, so that the support is still there at hour eight". That depends on three inputs:
- Arch type sets the target arch height. A flat foot has a bigger gap to fill; a high arch needs less lift and more cushioning.
- Body weight sets the shell firmness. Foam compresses in proportion to load, so heavier users need a firmer shell to hold the same shape.
- Primary use adjusts both. Eight hours standing on concrete is a harder test of compression resistance than a 30-minute daily walk.
Insole firmness reference — Shore hardness by insole class
Durometer (Shore) hardness measures resistance to indentation. Different scales cover different material ranges, which is why a "60" can mean very different things. Rough guide:
| Insole class | Typical shell material | Typical hardness | Fascia offload |
|---|---|---|---|
| Soft gel / cushion | Silicone gel, PU memory foam | Shore 00 ≈ 30–45 | Low |
| Comfort EVA | Single-density EVA | Shore A ≈ 45–55 | Low–moderate |
| Semi-rigid orthotic | Dual-density EVA | Shore A ≈ 55–70 | Moderate–high |
| Rigid orthotic | Polypropylene / TPU shell | Shore D ≈ 50–65 | High |
| Custom-milled | Milled EVA / composite | Firm, matched to gait | High (matched) |
As a very rough cross-scale conversion, Shore 00 ≈ Shore A + 40 at the soft end, and Shore D ≈ Shore A − 40 at the firm end. The Shore durometer scale is defined by ASTM D2240.
Arch height and heel cup depth by foot type
| Arch type | Target arch height | Heel cup depth | Priority |
|---|---|---|---|
| Low / flat (pes planus) | 28–34 mm | 18–25 mm | Fill the arch gap, control overpronation |
| Neutral / medium | 22–26 mm | 15–18 mm | Balanced support + comfort |
| High / cavus | 16–20 mm | 15–20 mm | Cushioning & shock absorption over lift |
A high arch is often rigid and does not want to be pushed up hard — too much arch height concentrates pressure at the apex and under the ball of the foot. A flat foot is the opposite case: it needs the tallest support and the deepest heel cup to resist collapsing inward.
Method: orthotic fitting norms + Shore durometer scale (ASTM D2240) · Deterministic calculation — no AI, no arbitrary estimate
Constants used
| Constant | Value | Basis |
|---|---|---|
| Arch height — low/flat | 28–34 mm | Tallest support tier; fills the larger medial gap in pes planus |
| Arch height — neutral | 22–26 mm | Mid support tier |
| Arch height — high | 16–20 mm | Low support tier; cavus foot prioritises cushioning |
| Shell tier — < 68 kg (150 lb) | Shore A 50–60 | Semi-rigid; light load compresses foam slowly |
| Shell tier — 68–91 kg (150–200 lb) | Shore A 60–70 | Firm; standard dual-density EVA range |
| Shell tier — > 91 kg (200 lb) | Shore A 70–80 / rigid TPU | Rigid; foam-only shells flatten under sustained heavy load |
| Use modifier — standing all day | +1 shell tier, add PU top layer | 8+ hr on hard floor is the hardest compression test |
| Use modifier — running | firm shell + forefoot cushioning, heel cup ≥ 20 mm | Repeated impact loading |
| Use modifier — recovery / in-house | −1 shell tier, plush top layer | Short durations; comfort over correction |
Calculation
weight_kg = unit === 'lb' ? weight / 2.2046 : weight
— Arch-support height (mm), from arch type —
arch_height = { low: [28,34], neutral: [22,26], high: [16,20] }[arch_type]
heel_cup = { low: [18,25], neutral: [15,18], high: [15,20] }[arch_type]
— Base shell tier, from body weight —
shell_tier = weight_kg < 68 ? 1 : weight_kg <= 91 ? 2 : 3
— Apply use-case modifier —
if use === 'standing' → shell_tier = min(shell_tier + 1, 3)
if use === 'recovery' → shell_tier = max(shell_tier - 1, 1)
— Map tier to Shore A range —
shore_A = { 1: [50,60], 2: [60,70], 3: [70,80] }[shell_tier]
General fitting guidance only — not a medical device recommendation. Systematic reviews find foot orthoses reduce plantar heel pain in the short to medium term, but do not establish one universal firmness. Persistent pain should be assessed in person.
From a target number to an insole you can actually buy
This calculator gives you a target: an arch height in millimetres and a Shore hardness band. The hard part is that almost no retail product page lists either number — you get "maximum support" and "premium cushioning" and a photo. Matching your target to a real insole means finding measured specs.
PlantarProof.com exists for that gap. Every insole, brace, and shoe it recommends is physically wear-tested, and the reviews publish the numbers this calculator asks you to match — measured arch height in millimetres, shell durometer, heel-cup depth, and how the foam held up after weeks of wear rather than on day one. A few ways the two line up:
- Flat feet + heavier build + standing all day: your result will call for a tall (30 mm+) rigid shell. Cross-check it against PlantarProof's best insoles for standing all day and flat-feet reviews, which report compression resistance after full shifts.
- High arch + running: your result de-prioritises lift and asks for forefoot cushioning — their shoe and high-arch insole picks note where the cushioning sits.
- "Is this drugstore gel insole firm enough?": their Dr. Scholl's review measures the actual durometer, so you can compare it against the Shore band you got above instead of guessing.
- Morning first-step pain: if that is your main symptom, an insole may not be the whole answer — their morning heel pain and night splint guides cover the overnight side.
- Not sure it is plantar fasciitis at all: their plantar fasciitis vs heel spur explainer walks through what imaging actually shows.
The site covers insoles, arch supports, night splints, braces, and shoes for plantar fasciitis, and buys every product it tests with its own money.
See measured insole reviews →
Frequently asked questions
Firm enough that it does not fully compress under your body weight through the day. Most functional shells sit in the Shore A 55–80 range; heavier users and all-day standing push toward the firm end or a rigid TPU/polypropylene shell. A soft gel insole that feels great in the store often bottoms out under load and offloads little of the fascia.
Low or flat feet generally need the tallest support, roughly 28–34 mm, to fill the larger gap under the medial arch and slow overpronation, plus a deeper 18–25 mm heel cup. Neutral arches sit around 22–26 mm; high arches usually need less lift, around 16–20 mm.
For plantar fasciitis the goal is to reduce strain on the plantar fascia, which comes mainly from arch support and a deep heel cup rather than from cushioning. A firm supportive shell with a thin comfort top layer typically offloads the fascia better than a thick soft insole that collapses under weight.
Yes. Foam and EVA compress in proportion to load, so a shell that holds its shape for a 60 kg person can flatten under a 100 kg person. Heavier users generally need a firmer shell tier, or a rigid shell material, to keep the same effective support through a full day.
Shore (durometer) hardness measures resistance to indentation, defined by ASTM D2240. Soft gels and memory foams are read on Shore 00 (about 20–60). Most insole shells and EVA foams are read on Shore A (about 45–90). Rigid polypropylene or TPU shells are hard enough to be read on Shore D (about 45–65).
A deeper heel cup cradles the fat pad under the heel and improves rearfoot stability. Around 15–18 mm is typical for a neutral setup; flat feet and heavier users often benefit from 18–25 mm. A shallow heel cup lets the heel pad splay under impact, which can worsen first-step pain.
No. It gives general starting targets from body weight and self-reported arch type. Persistent or severe heel pain, numbness, or pain that does not improve with support and stretching should be assessed in person, and custom orthoses may be prescribed after a gait assessment.