The FourSide is the tall, narrow, four-sided jar and the WildSide+ is the wider five-sided one with a sloping wall. The WildSide+ is the better all-rounder and is what the Classic 575 ships with; the FourSide suits smaller, thicker, single-serving loads because its narrow base forces a small quantity back onto the blade. Neither holds anywhere near the volume printed on it once liquid is involved.
The number that matters is maximum liquid fill.Blendtec publishes a maximum liquid fill of 36 oz for the WildSide+ jar on the Classic 575’s own product page. That is the figure to plan around — not the total vessel volume, which is what a jar’s name and its moulded markings refer to. Every capacity claim you read about these jars is one or the other, and most write-ups do not say which. This is the same total-volume-versus-usable-capacity problem we flag across the whole category in what size blender do I need.
What the two shapes are actually doing
The FourSide is tall and narrow with four straight walls.A narrow base means a small quantity still reaches the blade, and straight corners break the smooth spin that would otherwise let a load rotate as a block without being cut. That makes it good at thick, low-volume work — a single smoothie, a small batch of something dense, a quantity that would disappear into a wider jar.
The WildSide+ is wider with a fifth, sloping wall. The extra wall and the wider base are there to disrupt circulation more aggressively in a larger load, which is what a bigger batch needs: a wide jar that spins smoothly is a jar where the middle never gets cut. It handles more at once and it is the jar Blendtec pairs with the Classic 575.
Both work with the same blunt, unremovable blade, which is the design decision that lets Blendtec machines finish thick mixtures without a tamper at all. There is no tamper for either jar, and none is needed — that is the whole point of the approach, and it is the clearest difference between Blendtec’s answer and Vitamix’s. The side-by-side is in Blendtec vs Vitamix.
What each jar is for. Figures are Blendtec’s own.| Consideration | FourSide | WildSide+ |
|---|
| Shape | Tall, narrow, four straight walls | Wider, five walls, one sloping |
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| Best at | Small, thick, single-serving loads | Larger batches; the general-purpose jar |
|---|
| Tamper | None — blunt blade instead | None — blunt blade instead |
|---|
| Ships with the Classic 575 | No | Yes |
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| Published maximum liquid fill | Not published for this configuration | 36 oz |
|---|
The empty cell is deliberate. Blendtec publishes a maximum liquid fill figure for the WildSide+ on the Classic 575’s page and we quote it; we found no equivalent published figure for the FourSide in that configuration, so we leave it blank rather than infer one from the jar’s stated volume. An inferred number in a capacity table is exactly the kind of thing that gets repeated as fact, and our method does not allow it.
Which to buy
If you are buying a machine,take the WildSide+ — it is what the Classic 575 comes with and it is the better single jar for most households. There is no decision to make here unless you are buying a jar separately.
If you are adding a second jar, the FourSide earns its place only if you regularly blend small, thick quantities that get lost in the wider jar: one smoothie, a dressing, a small batch of something dense. If your loads are family-sized, a second WildSide+ is more useful than a FourSide.
If you are choosing between machines,the jar is not the deciding factor. The 36 oz maximum liquid fill is the real constraint on the Classic 575 as a whole, and it is the reason we rank it below bigger-batch machines on family-sized jobs. The other thing to know is the warranty: two years for this model, parts and labour — not the eight years the internet widely repeats, which belongs to Blendtec’s Designer and Professional lines. We tabulate every one of them in blender warranties compared.
Blendtec Classic 575 — the machine both jars fit
1,560 published watts, preset cycles, and a blade design that makes a tamper unnecessary. It is excellent at thick mixtures for its size and genuinely cabinet-friendly — the short base fits under a standard upper cabinet, which is a real constraint in more kitchens than spec sheets admit. The two limits to weigh are the 36 oz maximum liquid fill and the two-year warranty. Full detail in the Classic 575 review.
How we picked
Blenders we claim to have tested: 0
Every site in this category tells you they blended twenty smoothies. We have not blended any, and we say so. Here is what we do instead: read the manufacturer's manual and spec sheet, record the published motor rating, jar capacity and warranty term, verify the warranty against the manufacturer's own warranty document, work out what the machine costs to run for a year and show the arithmetic, and cite all of it so you can check the work. Prices come live from Amazon's API and disappear rather than go stale.
The scores above are judgments from that documented research — not test results. The full rubric, including what disqualifies a product, is published in full.
