diff --git a/src/CircleTool.cpp b/src/CircleTool.cpp
index e69de29..cdd3804 100644
--- a/src/CircleTool.cpp
+++ b/src/CircleTool.cpp
@@ -0,0 +1,259 @@
+#include "CircleTool.h"
+#include "ViewportWidget.h"
+#include "Camera.h"
+#include <QMouseEvent>
+#include <QKeyEvent>
+#include <QPainter>
+#include <QVector>
+#include <QOpenGLShaderProgram>
+#include <cmath>
+#include <QtMath>
+
+CircleTool::CircleTool(ViewportWidget* viewport)
+    : SketchTool(viewport)
+{
+}
+
+void CircleTool::activate()
+{
+    SketchTool::activate();
+    m_dimensionModes << "diameter";
+    m_dimensionPropertyNames["diameter"] = "diameterInput";
+
+    m_viewport->setProperty("diameterInput", "");
+    m_viewport->setProperty("dimensionEditMode", "diameter");
+}
+
+void CircleTool::mousePressEvent(QMouseEvent *event)
+{
+    gp_Pnt p;
+    if (!m_isDefining) {
+        if (m_viewport->isSnappingOrigin()) {
+            p.SetCoord(0, 0, 0);
+        } else if (m_viewport->isSnappingVertex()) {
+            p = m_viewport->snapVertex();
+        } else {
+            QVector3D worldPos = m_viewport->unproject(event->pos(), m_viewport->currentPlane());
+            p.SetCoord(worldPos.x(), worldPos.y(), worldPos.z());
+        }
+        m_centerPoint = p;
+        m_isDefining = true;
+        m_viewport->setProperty("diameterInput", "");
+        m_viewport->setProperty("dimensionEditMode", "diameter");
+    } else {
+        QVector3D worldPos;
+        QVector3D centerPos(m_centerPoint.X(), m_centerPoint.Y(), m_centerPoint.Z());
+
+        QString diameterInput = m_viewport->property("diameterInput").toString();
+        bool diameterFromInput = false;
+        double inputDiameter = 0;
+
+        if (!diameterInput.isEmpty()) {
+            bool ok;
+            inputDiameter = diameterInput.toDouble(&ok);
+            if (ok) diameterFromInput = true;
+        }
+
+        if (diameterFromInput) {
+            QVector3D mousePos = m_viewport->unproject(event->pos(), m_viewport->currentPlane());
+            QVector3D mouseDir = mousePos - centerPos;
+            if (mouseDir.lengthSquared() < 1e-9) {
+                if (m_viewport->currentPlane() == ViewportWidget::SketchPlane::XY || m_viewport->currentPlane() == ViewportWidget::SketchPlane::XZ) {
+                    mouseDir = QVector3D(1, 0, 0);
+                } else { // YZ
+                    mouseDir = QVector3D(0, 1, 0);
+                }
+            }
+            double radius = inputDiameter / 2.0;
+            worldPos = centerPos + mouseDir.normalized() * radius;
+        } else {
+            if (m_viewport->isSnappingOrigin()) {
+                worldPos.setX(0); worldPos.setY(0); worldPos.setZ(0);
+            } else if (m_viewport->isSnappingVertex()) {
+                worldPos = QVector3D(m_viewport->snapVertex().X(), m_viewport->snapVertex().Y(), m_viewport->snapVertex().Z());
+            } else {
+                worldPos = m_viewport->unproject(event->pos(), m_viewport->currentPlane());
+            }
+        }
+        p.SetCoord(worldPos.x(), worldPos.y(), worldPos.z());
+        emit m_viewport->circleAdded(m_centerPoint, p);
+        deactivate();
+    }
+}
+
+void CircleTool::mouseMoveEvent(QMouseEvent *event)
+{
+    // To be implemented
+}
+
+void CircleTool::finalizeCreation()
+{
+    QVector3D worldPos;
+    QVector3D centerPos(m_centerPoint.X(), m_centerPoint.Y(), m_centerPoint.Z());
+
+    QString diameterInput = m_viewport->property("diameterInput").toString();
+    bool diameterFromInput = false;
+    double inputDiameter = 0;
+
+    if (!diameterInput.isEmpty()) {
+        bool ok;
+        inputDiameter = diameterInput.toDouble(&ok);
+        if (ok) diameterFromInput = true;
+    }
+
+    if (diameterFromInput) {
+        QVector3D mousePos = m_viewport->unproject(m_viewport->currentMousePos(), m_viewport->currentPlane());
+        QVector3D mouseDir = mousePos - centerPos;
+        if (mouseDir.lengthSquared() < 1e-9) {
+            if (m_viewport->currentPlane() == ViewportWidget::SketchPlane::XY || m_viewport->currentPlane() == ViewportWidget::SketchPlane::XZ) {
+                mouseDir = QVector3D(1, 0, 0);
+            } else { // YZ
+                mouseDir = QVector3D(0, 1, 0);
+            }
+        }
+        double radius = inputDiameter / 2.0;
+        worldPos = centerPos + mouseDir.normalized() * radius;
+    } else {
+        worldPos = m_viewport->unproject(m_viewport->currentMousePos(), m_viewport->currentPlane());
+    }
+
+    gp_Pnt p;
+    p.SetCoord(worldPos.x(), worldPos.y(), worldPos.z());
+
+    emit m_viewport->circleAdded(m_centerPoint, p);
+    deactivate();
+}
+
+void CircleTool::paintGL()
+{
+    if (m_isDefining) {
+        QVector<GLfloat> vertices;
+        QVector3D worldPos;
+        QVector3D centerPos(m_centerPoint.X(), m_centerPoint.Y(), m_centerPoint.Z());
+
+        QString diameterInput = m_viewport->property("diameterInput").toString();
+        bool diameterFromInput = false;
+        double inputDiameter = 0;
+
+        if (!diameterInput.isEmpty()) {
+            bool ok;
+            inputDiameter = diameterInput.toDouble(&ok);
+            if (ok) diameterFromInput = true;
+        }
+
+        QVector3D mousePos = m_viewport->unproject(m_viewport->currentMousePos(), m_viewport->currentPlane());
+
+        double radius;
+        if (diameterFromInput) {
+            radius = inputDiameter / 2.0;
+        } else {
+            worldPos = mousePos;
+            if (m_viewport->isSnappingOrigin()) {
+                worldPos.setX(0); worldPos.setY(0); worldPos.setZ(0);
+            } else if (m_viewport->isSnappingVertex()) {
+                worldPos.setX(m_viewport->snapVertex().X()); worldPos.setY(m_viewport->snapVertex().Y()); worldPos.setZ(m_viewport->snapVertex().Z());
+            }
+            radius = (worldPos - centerPos).length();
+        }
+
+        const int segments = 64;
+        for (int i = 0; i < segments; ++i) {
+            double angle1 = i * 2.0 * M_PI / segments;
+            double angle2 = (i + 1) * 2.0 * M_PI / segments;
+
+            QVector3D p1, p2;
+
+            if (m_viewport->currentPlane() == ViewportWidget::SketchPlane::XY) {
+                p1.setX(centerPos.x() + radius * qCos(angle1));
+                p1.setY(centerPos.y() + radius * qSin(angle1));
+                p1.setZ(centerPos.z());
+                p2.setX(centerPos.x() + radius * qCos(angle2));
+                p2.setY(centerPos.y() + radius * qSin(angle2));
+                p2.setZ(centerPos.z());
+            } else if (m_viewport->currentPlane() == ViewportWidget::SketchPlane::XZ) {
+                p1.setX(centerPos.x() + radius * qCos(angle1));
+                p1.setY(centerPos.y());
+                p1.setZ(centerPos.z() + radius * qSin(angle1));
+                p2.setX(centerPos.x() + radius * qCos(angle2));
+                p2.setY(centerPos.y());
+                p2.setZ(centerPos.z() + radius * qSin(angle2));
+            } else { // YZ
+                p1.setX(centerPos.x());
+                p1.setY(centerPos.y() + radius * qCos(angle1));
+                p1.setZ(centerPos.z() + radius * qSin(angle1));
+                p2.setX(centerPos.x());
+                p2.setY(centerPos.y() + radius * qCos(angle2));
+                p2.setZ(centerPos.z() + radius * qSin(angle2));
+            }
+            vertices << p1.x() << p1.y() << p1.z();
+            vertices << p2.x() << p2.y() << p2.z();
+        }
+
+        m_viewport->shaderProgram()->setUniformValue(m_viewport->colorLoc(), QVector4D(1.0f, 1.0f, 0.0f, 1.0f));
+        m_viewport->vbo().bind();
+        m_viewport->vbo().allocate(vertices.constData(), vertices.size() * sizeof(GLfloat));
+        glDrawArrays(GL_LINES, 0, segments * 2);
+    }
+}
+
+void CircleTool::paint2D(QPainter& painter, const QMatrix4x4& modelView, const QMatrix4x4& projection)
+{
+    if (m_isDefining) {
+        QVector3D worldPos;
+        QVector3D centerPos(m_centerPoint.X(), m_centerPoint.Y(), m_centerPoint.Z());
+
+        QString diameterInput = m_viewport->property("diameterInput").toString();
+        bool diameterFromInput = false;
+        double inputDiameter = 0;
+
+        if (!diameterInput.isEmpty()) {
+            bool ok;
+            inputDiameter = diameterInput.toDouble(&ok);
+            if (ok) diameterFromInput = true;
+        }
+
+        QVector3D mousePos = m_viewport->unproject(m_viewport->currentMousePos(), m_viewport->currentPlane());
+        QVector3D edgePos;
+        double diameter;
+
+        if (diameterFromInput) {
+            diameter = inputDiameter;
+            QVector3D mouseDir = mousePos - centerPos;
+            if (mouseDir.lengthSquared() < 1e-9) {
+                if (m_viewport->currentPlane() == ViewportWidget::SketchPlane::XY || m_viewport->currentPlane() == ViewportWidget::SketchPlane::XZ) {
+                    mouseDir = QVector3D(1, 0, 0);
+                } else { // YZ
+                    mouseDir = QVector3D(0, 1, 0);
+                }
+            }
+            edgePos = centerPos + mouseDir.normalized() * (diameter / 2.0);
+        } else {
+            edgePos = mousePos;
+            if (m_viewport->isSnappingOrigin()) {
+                edgePos.setX(0); edgePos.setY(0); edgePos.setZ(0);
+            } else if (m_viewport->isSnappingVertex()) {
+                edgePos.setX(m_viewport->snapVertex().X()); edgePos.setY(m_viewport->snapVertex().Y()); edgePos.setZ(m_viewport->snapVertex().Z());
+            }
+            diameter = (edgePos - centerPos).length() * 2.0;
+        }
+
+        painter.setRenderHint(QPainter::Antialiasing);
+        QFontMetrics fm(painter.font());
+
+        // Diameter dimension
+        QVector3D diameterTextPos3D = (centerPos + edgePos) / 2.0f;
+        QVector3D screenPosD = m_viewport->project(diameterTextPos3D, modelView, projection, m_viewport->rect());
+        if (screenPosD.z() < 1.0f) {
+            QString diameterText = diameterFromInput ? diameterInput : QString::number(diameter, 'f', 2);
+            QRect textRect = fm.boundingRect(diameterText + "__");
+            textRect.moveCenter(screenPosD.toPoint());
+            if (m_viewport->property("dimensionEditMode").toString() == "diameter") {
+                painter.fillRect(textRect.adjusted(-4, -2, 4, 2), QColor(64, 128, 255));
+            } else {
+                painter.fillRect(textRect.adjusted(-4, -2, 4, 2), QColor(50, 50, 50));
+            }
+            painter.setPen(Qt::white);
+            painter.drawText(textRect, Qt::AlignCenter, diameterText);
+        }
+    }
+}
diff --git a/src/CircleTool.h b/src/CircleTool.h
index e69de29..4d112ed 100644
--- a/src/CircleTool.h
+++ b/src/CircleTool.h
@@ -0,0 +1,28 @@
+#ifndef CIRCLETOOL_H
+#define CIRCLETOOL_H
+
+#include "SketchTool.h"
+#include <gp_Pnt.hxx>
+
+class CircleTool : public SketchTool
+{
+    Q_OBJECT
+public:
+    explicit CircleTool(ViewportWidget* viewport);
+
+    void mousePressEvent(QMouseEvent *event) override;
+    void mouseMoveEvent(QMouseEvent *event) override;
+
+    void paintGL() override;
+    void paint2D(QPainter& painter, const QMatrix4x4& modelView, const QMatrix4x4& projection) override;
+
+    void activate() override;
+
+protected:
+    void finalizeCreation() override;
+
+private:
+    gp_Pnt m_centerPoint;
+};
+
+#endif // CIRCLETOOL_H