diff --git a/common/calculate-swap3.ts b/common/calculate-swap3.ts
index a2f957a8..a40fe54a 100644
--- a/common/calculate-swap3.ts
+++ b/common/calculate-swap3.ts
@@ -89,6 +89,7 @@ export function calculateLPCost(
   deltaL: number
 ) {
   // TODO: this is subtly wrong, because of rounding between curTick and sqrtPrice
+  // Also below in buyYES
   const upperTick = Math.min(maxTick, Math.max(minTick, curTick))
   const costN = toRatio(upperTick) ** 0.5 - toRatio(minTick) ** 0.5
 
@@ -101,6 +102,62 @@ export function calculateLPCost(
   }
 }
 
+// Returns a preview of the new pool + number of YES shares purchased.
+// Does NOT modify the pool
+// Hm, logic is pretty complicated. Let's see if we can simplify this.
+export function buyYes(
+  pool: Swap3Pool,
+  amount: number // In M$
+) {
+  const tickStates = sortedTickStates(pool)
+  let tick = pool.tick
+  let stateIndex = 0
+  let amountLeft = amount
+  let yesPurchased = 0
+  while (amountLeft > 0) {
+    // Find the current tick state
+    while (tick >= tickStates[stateIndex + 1].tick) {
+      stateIndex++
+      if (stateIndex > tickStates.length - 2) {
+        // We've reached the end of the tick states...
+        throw new Error('Ran out of tick states')
+      }
+    }
+    const state = tickStates[stateIndex]
+    const nextState = tickStates[stateIndex + 1]
+
+    // Copied from above; TODO extract to common function
+    const noCost = toRatio(nextState.tick) ** 0.5 - toRatio(tick) ** 0.5
+    const yesCost =
+      1 / toRatio(tick) ** 0.5 - 1 / toRatio(nextState.tick) ** 0.5
+
+    if (noCost * state.liquidityGross <= amountLeft) {
+      // We can fully purchase up until the next tick state
+      amountLeft -= noCost * state.liquidityGross
+      yesPurchased += yesCost * state.liquidityGross
+      tick = nextState.tick
+    } else {
+      // Buy as much as we can at the current tick state. Derivation:
+      // noCostLeft = toRatio(upTick) ** 0.5 - toRatio(tick) ** 0.5
+      // (noCostLeft + toRatio(tick) ** 0.5) ** 2 = toRatio(upTick)
+      // TODO check flooring done here
+      const noCostLeft = amountLeft / state.liquidityGross
+      const finalTick = fromRatio((noCostLeft + toRatio(tick) ** 0.5) ** 2)
+      const yesCostLeft =
+        1 / toRatio(tick) ** 0.5 - 1 / toRatio(finalTick) ** 0.5
+
+      amountLeft = 0
+      yesPurchased += yesCostLeft * state.liquidityGross
+      tick = finalTick
+    }
+  }
+
+  return {
+    newPoolTick: tick,
+    yesPurchased,
+  }
+}
+
 // Currently, this mutates the pool. Should it return a new object instead?
 export function addPosition(
   pool: Swap3Pool,
@@ -165,5 +222,9 @@ export function toProb(tick: number) {
 // Returns the tick for a given probability from 0 to 1
 export function fromProb(prob: number) {
   const ratio = prob / (1 - prob)
+  return fromRatio(ratio)
+}
+
+function fromRatio(ratio: number) {
   return Math.floor(Math.log(ratio) / Math.log(1.0001))
 }
diff --git a/web/pages/swap.tsx b/web/pages/swap.tsx
index edeb6c48..6e5aeb7b 100644
--- a/web/pages/swap.tsx
+++ b/web/pages/swap.tsx
@@ -1,5 +1,6 @@
 import {
   addPosition,
+  buyYes,
   calculateLPCost,
   fromProb,
   getSwap3Probability,
@@ -136,11 +137,14 @@ export default function Swap() {
     tickStates: [],
   }
   INIT_POOL = addPosition(INIT_POOL, -(2 ** 23), 2 ** 20, 100)
+  INIT_POOL = addPosition(INIT_POOL, fromProb(0.32), fromProb(0.35), 100)
   INIT_POOL = grossLiquidity(INIT_POOL)
+
   const [pool, setPool] = useState(INIT_POOL)
 
   const [minTick, setMinTick] = useState(0)
   const [maxTick, setMaxTick] = useState(0)
+  const [buyAmount, setBuyAmount] = useState(0)
 
   const { requiredN, requiredY } = calculateLPCost(
     pool.tick,
@@ -149,6 +153,8 @@ export default function Swap() {
     100 // deltaL
   )
 
+  const { newPoolTick, yesPurchased } = buyYes(pool, buyAmount)
+
   return (
     <Col className="mx-auto max-w-2xl gap-10 p-4">
       {/* <BalanceTable /> */}
@@ -200,10 +206,20 @@ export default function Swap() {
       <Col>
         Bob: Buy Tokens
         {/* <input className="input" placeholder="User" type="text" /> */}
-        <input className="input" placeholder="Amount" type="number" />
+        <input
+          className="input"
+          placeholder="Amount"
+          type="number"
+          onChange={(e) => setBuyAmount(parseFloat(e.target.value))}
+        />
+        <Row className="gap-2 py-2">
+          <div>Y shares purchaseable: {yesPurchased.toFixed(2)}</div>
+          <div>New Tick: {newPoolTick}</div>
+          <div>New prob: {formatPercent(toProb(newPoolTick))}</div>
+        </Row>
         <Row className="gap-2">
           <button className="btn">Buy YES</button>
-          <button className="btn">Buy NO</button>
+          {/* <button className="btn">Buy NO</button> */}
         </Row>
       </Col>
     </Col>